How could one (and should one) convert someone from pseudoscience?

The one thing I've come up with is to somehow introduce them to classical logical fallacies.

That seems extremely dangerous. Most of the time, this will just make people better at rationalization, and many things that are usually considered fallacies are actually heuristics.

1. Ask them the same question I suggest you ask yourself to be less wrong: "what evidence or argument would convince you this specific belief is in error or inconclusive?" If the answer is 'I don't know' then consider finding out. If the answer is 'none' then it's not an evidence or argument based choice so evidence or argument won't change anything. If the answer is 'this evidence / argument' then either the evidence / argument is there or it isn't. If it isn't then the claim is provisionally true, and in no way off limits to further questioning.

2. Decide if your are talking to your friend or if you are talking to the abstraction of 'pseudoscience' and don't confuse the two.

While happy thoughts seem exceedingly unlikely to by themselves cure any sort of case of cancer, certainly persistent unhappiness can lead to many physiological changes which are associated with degenerative disease and cancer risk (stress, trouble sleeping, and so on).

As Lumifer pointed out, it's important to consider what the practical consequences are of their beliefs. If the person you're referring to simply believes that engendering a sustainably happy state of mind will decrease their cancer risk, then I doubt there's anything to worry about. But if you would expect them to refuse a proven surgical technique and instead attempt to cure themselves by hanging out with their friends and watching fun movies, then surely it would be a highly useful service to this individual to point them in a better ideological direction.

Don't introduce them to a catalog of logical fallacies. Understanding a few important logical fallacies can help people who possess a propensity to propagate new conceptions through their web of beliefs, figuring out which beliefs should stay and which should go based on their new theory. But most people don't operate in this way. Updating all the different areas of one's belief structure in accordance to a newly acquired abstract tool doesn't come naturally to most individuals. If you coax a friend down the path toward Less-Wrong-style rationality, then there may come a day where reading 37 Ways Words Can Be Wrong would be quite an enlightening experience for them. But that day is probably not today.

I wonder whether your impression that the world of pseudo-science is a rosy one, and rationality is a window into the bleak reality of human life, is the key to the frustration you're communicating here. The only language that your non-rationalist friends will appreciate is the language of concrete results. If you can't employ your ability to think rationally to become noticeably better than them at activities they pursue in a serious way, give them health advice which to them seems to miraculously clear up certain long-standing inconveniences in their life, etc., then you're not giving them any evidence that your way of thinking is better than theirs.

Use your capacity for rational thinking to succeed in concrete endeavors, and then demonstrate to them the results of your competence. One day they may ask to look under the hood--to see the source of your impressive abilities. And then the time will have come to introduce them to the abstract rationality concepts you consider important.

This topic is valuable me. Every time when me and my girlfriend getting involved in argument it ends up badly and we continue on holding prior beliefs. She blames me for being too rational (In hollywood sense obv, none of my efforts to convince that the word has different meaning payed off). She is absolutely sure that when you hold a skeptical position, you are getting trapped into it, so entry gate for outside view is closed and you are limiting the spectrum of possibilities. With this I'm okay because I like to think of myself as an open-minded person and I can assume that the universe is not objective. But I'm rapidly getting tired when it comes to secret meanings of events and perfect casuality and total misunderstanding of randomness. In her head there is a strange mash of esoterics, buddhism, astrology somehow combined with evolutionary biology in perfect harmony. I'm not sure whether I should try harder to convert her or give up. Maybe I gave up already, because it seems that open-minded people lack neuroplasticity

Remember that a little bit of rationality in gullible hands can have unpleasant consequences. Next thing you know, they'll be the ones accusing you of being unreasonable.

There is a range of possible courses of action open to you. On one end, you can happily live with your friends' strange ideas and not interfere at all, even when they teach the same ideas to other people or to their children. On the other end, you can make new friends whose ideas are closer to yours and let your old friends fade from your life. My personal experience has been a case-by-case mix of both approaches.

It seems likely to me that happy thoughts would in fact reduce the rate of death by cancer, although not by very much.

The discussion so far has all been hypothetical. Does anyone who has been in this situation (on either side) have any case studies to offer? I have none.

Possibly asking something like "you're good at finding points that back up your beliefs, but you also need to spend time thinking about points that might contradict your beliefs. How many contradictory points can you think of over the next five minutes?"

One approach may be to see if you can find the scientific research that some of the hype is extrapolated from, and discuss that. In the case of Bruce Lipton, that may mean finding and discussing scientific papers about epigenetics and about the effects of low-level magnetic fields on biological systems. If you read the actual papers, and understand them enough to explain them to someone with less of a scientific background, then that could be a starting point for discussing the topics with your friends.

I'm not sure if that will help. But many things that look like pseudoscience have something real that is related to them. Talking about the real things can help separate them out from the bullshit.

Science journalism in general is pretty terrible. Someone has to go way beyond what is offered by mainstream media to have any kind of clue. It's a lot of work, and a lot to expect the average person to do, especially when so many scientific journals are paywalled.

Instead of attempting to shatter the illusion, one thing to do may be to demonstrate techniques for dealing with things when they are uncertain, and out of one's control. If it is a psychological crutch, then having techniques to replace it, rather than a new vision of the world, may remove the need for the crutch.

Some people are more open to different points of view than others. I would start with the people who are at least somewhat open to considering other ideas. And also be prepared to listen to them and find out what it is that they think is important about their beliefs. You may share more common ground than you think. Or they may have had personal experiences that are extremely different from yours. You can probably learn something.

My own impression is that much of pseudoscience is framed in such a way that it cannot be disproven (falsifiability). I once had a friend discuss chakra networks with me. At one point he said: "don't you think it's possible for the human body to exchange energy with the outside world?" This is technically true. Philosophy can sometimes make some very ridiculous arguments too, but those cannot be disproven either. I would instead recommend explaining scientific philosophy and why this has proven so successful.

In general you have to keep in mind that if you challenge someone's believe but don't convince them that their beliefs are wrong there a good chance that you increase the strength of their beliefs.

should I do anything about it?

Are there any practical consequences of these beliefs? As long as they are not telling cancer patients to skip the therapy and think happy, I don't see any harm. Trying to fix other people's beliefs just because you don't like them seems to be... not a terribly productive thing to do.

that also means that I'll be standing idly by and allowing bullshit to propagate

Have you looked at a TV screen recently..?

Let's try a simpler question: What should you tell someone who's spending money on bogus "natural remedies" that can't possibly work?