Instructions
Turn on the Xbox 360 system and press the center button on the Xbox 360 controller.
Navigate to the "Settings" section of the Xbox Guide and scroll down to the "Account Management" option. Press the "A" button to select the Account Management option.
Navigate to the "Your Information" pane on the Account Management screen. Scroll down to the "Change Windows Live ID" option and press the "A" button.
Select the "Yes" option when asked if you have another Windows Live ID to switch to. Enter the password associated with your current email and select the "Sign In" option to sign in to your current account.
Type the email address and password for the Windows Live ID account you want to use instead of your current email address. Select the "Sign In" option to sign in to the new account.
Select the "Yes, Change" option, followed by the "Done" option to erase the Gamertag's current email address and replace it with the new email address.