I don't think you can, once you've signed up.
To be a different gender, you'd need a new account, but you'd have to make sure you got staff to delete your current account BEFORE making a new one. Having two would obviously be seen as cheating, and you could be banned by staff.
Otherwise, you may just have to live with the wrong gender on Res :( Perhaps they should consider changing this, I mean, what if someone actually does change gender during their time on Res?
Anyway, that's kinda irrelevant, sorry :P
Hope this helps.