Follow these steps to remove your Apple Watch from a device / Apple ID, even if you don't have it anymore!
This allows allows you to make sure that the watch that you're about to buy from someone else is ready for you to use.
- Open the Apple Watch app on your iPhone and tap the My Watch tab.
- Tap your watch's name at the top of the screen, then tap the information button "i"
- Tap Unpair Apple Watch. For cellular models of Apple Watch, tap Remove Carrier Plan. If you aren't going to pair with a different watch or iPhone, contact your carrier to cancel your cellular subscription
- Enter your Apple ID password to turn off Activation Lock
- Tap again to confirm - When you unpair, your iPhone will attempt to backup your watch so when you get a new Apple Watch, you can use the backup to set it up!