Instructions
Turn on the Xbox 360 game console. Press the "Guide" button, followed by the "X" button, and select the gamertag you want to modify to sign in.
Scroll down to the "My Xbox" section of the system's Dashboard, highlight your gamertag and press the "A" button.
Scroll down to the "Change Gamertag" option and press the "A" button.
Highlight the "Enter New Gamertag" option and press the "A" button.
Make any modifications to your gamertag on the following screen and press the "Start" button to save the changes.