What would you do if you were in his situation?

Firstly, lets say you're a boy was called Tate and the girl is called Amanda. In 2012, You and Amanda are good friends, you're not sure if you she likes you but you don't like her (in a romantic way). Anyways, so Amanda asks you if you want to go prom with her and you say yes. It's 2013 now and you and this girl called Lana have liked each other for a couple of months. You've been close for a while and you're basically going out; and you want to go prom with Lana obviously instead of Amanda but you don't know if you should tell Amanda or how. What do you do? Example; tell Amanda, just go with Amanda etc? Thank you :)