Instructions
Open your Web browser via the Start menu. Navigate to Xbox.com.
Click "Sign In" in the upper-right-hand corner of the Xbox site. Enter your Windows Live ID email and password to log in.
Click your Xbox Live gamertag in the upper-right-hand corner. Click "Edit Profile" to open the advanced profile editor.
Click "Change gamertag." Enter the new gamertag.
Click "Change gamertag," then "Save." Your Xbox Live gamertag has been successfully modified. You also have the option of modifying your motto, name, location and bio.