Coming out of a relationship is hard. Its even harder when youve left a girl seeing you as a beta-male. Any girl seeing you as a beta-male is a bad thing let alone a girl who is in the same social circles as you. Especially if youve broken up with them.

Now my question is how does one go about changing an ex-girlfriends view of oneself?

In my mind all you can really do is:
Appear un-harmed, like her leaving just gave you the right to bang better girls.
Do new things, make it look like your life is incredible without her.
Date lots of women.
Make new friends.
Look better, visit the gym more, eat better.
Read, get better with words. Be good at talking.

The thing is unless you mention these things she will most likely not notice. Is there anyway you can come across as Alpha in the brief moments you might have when you bump into her? Coming across as a confident individual with a fantastic life is hard to achieve when you only see them briefly.

Any ideas?

you cant change her view, unless you change yourself first. But you answered your own topic anyway.

All of that is just window dressing if the underlying motive (to want to get better for one's self) hasn't changed. Doing all the aforementioned things to win one over is akin to throwing a fresh coat of paint on a house with a rotted-out interior.