There are few reasons why you would need to factory reset your Xbox. You may be selling it or have some issues with it that a factory reset may help with. This is how you factory reset your Xbox and remove it from your devices.
Step 1: Press the Guide Button
Step 2: Go to Settings
Step 3: Got to System, Console Info
Step 4: Click Reset console
Step 5: Select desired option (‘Reset and Remove Everything’ or ‘Reset and Keep Games & Apps’)
The console will then turn off. If you are keeping your console just restart it and setup your account like brand new. If you are selling your console you are ready to go.