Instructions
Turn on your Xbox 360 console.
Press the Xbox 360 logo button on the center of your console to open the guide.
Press "Right" on your controller twice and go to the "Settings" tab. Select "Account Management."
Select "Windows Live ID" from the menu and select "Change Windows Live ID."
Select "Yes, I do" and then enter the new Windows Live ID email address. This will set the email address as the primary link on your gamertag.