Instructions
Power on the Xbox. Sign out of any Xbox Live accounts, if signed in. Do this by pressing "Guide," then "X" on the controller.
Press the "Guide" button on the controller and select "Recover Gamertag." Select "Recover Gamertag" again.
Enter your Windows Live ID account information associated with your Xbox Live gamertag. Follow the remaining instructions on-screen to recover your profile and settings to the Xbox game console.
Navigate to the "My Xbox" menu, select your recovered gamertag and select "Change Gamertag."
Select "Enter New Gamertag," enter the modified gamertag and click "Yes, Use This Gamertag" to confirm the changes.