How to change one's bad temper?

One's temper is very difficult to change unless you are a quite clever person.And you know your friend very well,and you have many ways to solve problems.But if your friend himself does not want to change his personality,all the things you did is useless.By the way,if you are a girl,and your friend is your boyfriend,do not try to change him.It is nearly is impossible unless you are so important for him,and you will pay out a lot.