Change their character, do you mean? That's a very difficult question ... But it is sure that people can change what they do and how they react to events, and if you change your behaviour it does eventually have an effect on your character.
For example: Let's say you are very negative generally. You always suspect the worst and find it hard to trust or enjoy things. Well, you can't just wake up and say 'I'm now a happy, positive person' but you can make a real effort, when you find yourself thinking about a new project etc 'Oh, I may as well not do it because it will never work' to say 'Maybe, but I'm going to do it anyway.' And if you always suspect people's motives you can't help that, but sometimes you can say 'I'm going to see how this person behaves, and if they give me no reason to mistrust them, I won't do so.'
So I would say: Act as if you were the kind of person you admire, and that will bring you much closer to becoming that person (and the people you admire probably needed to make an effort to get that way too).