How can I change my tag and/or username on this site?
Your Waypoint username is directly linked to your Xbox gamertag; to change it here you need to change your gamertag using one of the methods in this link.
If you change your gamertag, you have to play the games again to reflect your progress, just join in your games again, that was a problem here
Go on to your Xbox one or 360 and choose settings and go on to change name, doing this will make your halo waypoint account the same name as your xbox as they are linked hope this helps.