How do I unpair my Apple Watch?

You can unpair the Apple Watch by using the WatchApp on your paired iPhone, or erase the watch directly.

But, if you do that without the iPhone, the Activation Lock will not be removed.

Have a look at the related article:

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Unpairing erases your Apple Watch, restores it to factory settings, and removes Activation Lock. If you don't have the iPhone that's paired with your watch, you can erase the watch, but Activation Lock will remain.

1. Keep your Apple Watch and iPhone close together.
2. On your iPhone, open the Apple Watch app.
3. Go to the My Watch tab and tap All Watches.
4. Tap the info buttonnext to the watch that you want to unpair.
5. Tap Unpair Apple Watch.
6. Tap Unpair [your Apple Watch name].
7. If your watch has a cellular plan, choose whether to keep or remove it.

• • If you'll pair your watch and iPhone again, keep your plan.
  • If you're selling or giving away your watch, remove your plan. Contact your carrier if you want to cancel your cellular subscription.

8 . Type your Apple Account password to disable Activation Lock, then tap Unpair.

After it unpairs, you can set up your Apple Watch again or turn it off if you're selling it or giving it away.

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If you don't have your iPhone, erase your Apple Watch

This erases your Apple Watch and restores it to factory settings, but doesn't remove Activation Lock. If you're selling or giving away your Apple Watch, it's best to use your iPhone to unpair it

1. On your Apple Watch, open the Settings app.
2. Tap General, then tap Reset.
3. Tap Erase All Content and Settings.
4. Type your passcode, if prompted.
5. If your watch has a cellular plan, choose whether to keep or remove it.

• • If you'll pair your watch and iPhone again, keep your plan.
  • If you're selling or giving away your watch, remove your plan. Contact your carrier if you want to cancel your cellular subscription.

6 . Tap Erase All to confirm.

After it erases, you can set up your Apple Watch again. You'll need the Apple Account and password associated with the watch to turn off Activation Lock.

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copied from: Unpair and erase your Apple Watch - Apple Support