I don't think you can. I think that the only thing you can do is just wait it out. If the two ever break up, be there for her, be her friend. Maybe she'll see you as more than just a friend if you help her out a lot.
They won't stay together long, if they have a messy break up you can be there to comfort her and there is a good chance she will hook up with you. Or you can spend time with her, appear sweet,...
first thing don't freak out. these things are very common at teenage. try to know indirectly whether how she treats you (as a "just friend",best friend or more than that.) if she likes u then u can...
sorry to say but if she is in a loving relationship there is not much you can do. breaking her and her boyfriend up would be selfish on your part and also really mean and likely to make her very mad...