Probably in the Live Market Place. There should be something about editing your account. But I sent my 360 in for repairs so I can't check to give you a more detailed explanation.
Mod Edit: Image removed. Some users may find it offensive and inappropriate.
SSBB FC: ????-????-???? (New Wii, so I gotta check)
Been playing alot of WarCraft III lately, anyone up for game? Just message me, my account is the same as here and GT.