Instructions
Turn on your Xbox 360 and log in to your GamerTag.
Go to the "My Xbox" area in the Dashboard menu.
Scroll right and select your avatar.
Scroll right to the "Account Management" section.
Select "Manage Account."
Scroll right to the "Your Information" section.
Select "Windows Live ID."
Select "Change Windows Live ID," then select "Yes, I do," indicating you have another Windows Live ID that you want to use.
Enter your current Windows Live ID password and select "Sign in."
Enter the email and password linked to the new Windows Live ID. Select "Sign in."
Select "Yes, Change" to confirm the Windows Live ID change. Click "Done" to complete the process.