How do I reset the watch back to factory settings?

The devices do 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 your round data then you are all set to pass it on.

All your data is uploaded into your account from the watch when you sync and is then removed from the watch. If you are giving your watch to a new user, the courses you have downloaded to the watch (V2 only, V3 courses are preloaded) 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 as they will have to create a new account and assign the tags to their own clubs in My Bag before playing.