Instructions
Press the "Guide" button on the Xbox 360 controller to power on the Xbox 360 console and controller.
Navigate to "My Xbox" when the system boots up. Select your current Xbox Live gamertag. You may have to sign in (by providing your password) if you are not configured to log in automatically.
Click "Change Gamertag." Click "Enter New Gamertag." Enter the gamertag you want to use. Make note of the listed price; in some instances, it may cost you Microsoft Points to make the change. Microsoft typically allows one free change. Subsequent changes cost roughly 800 Microsoft Points at the time of publication. If you have no Microsoft Points currently credited to your account, you will be asked to enter your payment information.
Click "Yes, use this gamertag" to save the changes.