It’s your online alter-ego for Xbox. Your gamertag represents you when you’re playing games and sharing with other people in the Xbox community. We create a gamertag for you the first time you sign in to Xbox. If you don’t like it, you get to change it once for free.
To change your gamertag in Windows 10, open the Xbox app and select your gamerpic to open your profile. Then go to Customize > Change gamertag and follow the instructions.
To change it on your Xbox One, press the Xbox button on your controller to open the guide, select your gamerpic, and then select My profile > Customize profile. Select your gamertag and either pick a suggested one or make your own.