There's no one answer to that, because "how" is dependent on the individual and their relation to their thoughts (i.e. my thoughts are probably organized very d

There's no one answer to that, because "how" is dependent on the individual and their relation to their thoughts (i.e. my thoughts are probably organized very differently from yours). Furthermore, it is hard to claim that one would be able to "alter their thoughts" as directly as your sentence suggests, but meditation at least allows the recognition of separation between the "worry" in the mind and the "problem" in reality. For some people, this recognition leads to a sense of control over that worry, which might be one example of your "how".