My friend is a bullying *****, how can I change her?

You need to be honest with her. You should just tell her that you don't want to be friends with her anymore, you don't have to go into detail why but just tell her that she's let you down and you just don't have it in you to be friends with her anymore. It's almost impossible to change people, only they can change themselves. Just let her go, find yourself friends who are similar to your character/personality and try not to be bitter either. Good luck!