Instructions
Turn on your Xbox 360.
Scroll to the "My Xbox" menu.
Navigate to the "Sign In" panel and then press "A" to select the avatar associated with your Xbox Live Gamertag.
Scroll to the "My Xbox" menu again and then navigate to your avatar panel and press "A" to select it.
Scroll to the "Account Management" panel and then press "A" to select "Manage Account."
Scroll to the "Your Information" panel and then press "A" to select the "Windows Live ID" option.
Press "A" to select the "Change Windows Live ID" option and then press "A" to select the "Yes" option. If you already have a Windows Live ID associated with your Gamertag and are simply attempting to change it, you will also need to supply the password for your current Windows Live ID.
Type the email address for your Windows Live ID into the "Email Address" field and then type the password into the "Password" field.
Scroll to "Sign In" and then press "A."
Press "A" to select the "Yes, Change" option.