Can't log into Facebook because I'm not allowed to edit my Email address.

So this is a little problem I just came across.

I switched my email a few months ago and my xbox gamertag is hooked up to that email - but whenever I try to log into Facebook on the Xbox my old email is still being displayed. That wouldn't bother me so much if Microsoft didn't attempt to be smart and keep my email there; thinking I won't need to change it.

So the question is; how can I change it?

Many thanks

- Martin