Then the scientists will try to come up with a better one. This is actually how science makes progress. Our theories are never perfect, and sooner or later someone will do an experiment that shows that the theory was wrong. Then we have to find a new theory that explains the new experiment and also all the existing ones. This way understand nature better and better.
Firstly, you have to make sure that your argument for why they are wrong is very strong (you have to have strong data and a heavy conclusive result or a really good mathematical proof). Then, you just need to get your ideas published so that other people can see them. It’s just like Arttu says, this is what keeps us progressing in science. Our ultimate goal is to get the best scientific explanations that we can and if someone thinks a theory is wrong then what they have to say might help us get to the correct answer so it’s a very welcome contribution to the literature.