There is this girl I met. She had a boyfriend who she had been dating for two years. He promised to be better and all this other stuff. In fact this has happened multiple times to her. What are some ways I could try to win her back?

You don't. She has a guy already.

There is no magic way of 'winning someone back'. All you can do is tell her how you feel and see what happens. She is an adult herself and can make up her own mind. If she doesn't know how you feel, tell her as she could feel the same way but be afraid of rejection. Alternatively she may not like you in that way anymore. Either way you won't know until you speak to her.

If you decide to do this, be prepared for the possibility of an answer you don't want to here. You need to decide if you want to take that risk. If you don't, then you need to leave her to it.

Hi, can you please elaborate your question, Do you want to get her in your life or you want that guy to be improved. As your question says How do I win her back, it seems that you wre having some relationship with her?