You can find your username through your watch menu in-game.
- Open your watch, navigate to your profile (top left on home menu)
- Click "Settings"
- Click "Account Settings"
- You can find your username listed under "@AccountName"
Keep in mind that your username is not the same as your display name. Display names are not unique and can be changed up to three times each week. All usernames are unique and you can only change your username once every 180 days. Your username will always start with "@".
If you're looking to change your @username instead, see this page for more info.