The watch does not have a factory reset as such. If you are looking to sell your watch or give it to someone you know then as long as you have synced the round data from the watch then you are all set to pass it on.
All your data is uploaded into your account from the watch when you sync and removed from the watch. If you are giving your watch to a new user, the courses on the watch will be deleted as soon as the new owner syncs their selected courses to the watch.
Just make sure to fully sync your watch a final time before passing it on. The old tags will work fine for the new user as well.