How do I unpair my Apple Watch from an iPhone I no longer have?

See: How to pair your Apple Watch with a new iPhone - Apple Support

If you don't have your old iPhone or you erased it

Your Apple Watch might still be paired with your old iPhone. Use the steps below to unpair:

1. Erase your Apple Watch. 
2. Set up your new iPhone and sign in to iCloud. If you've already set up and signed in, skip this step.
3. Open the Apple Watch app on your new iPhone, then pair your watch with your new iPhone.
4. Try to restore from a backup. Make sure that your iPhone and Apple Watch are up to date; if they're not, you might not see your backup in the list. You will lose Activity, Workouts, or settings on your watch that changed between the last backup and now. If you use iCloud in iOS 11 and later, Health and Activity data is stored in iCloud and automatically kept up to date on devices where you're signed in with the same Apple ID. 

• • If you’re asked to restore from a watch backup, select the most recent backup. 
  • If you don't have a backup, you need to set up your Apple Watch as new. 

1. Follow the onscreen steps to finish setting up.
2. Start using your Apple Watch with your new iPhone. You might need to wait a few days for all your iCloud data to appear.

To unpair and erase watch and remove Activation Lock

From a web browser,sign in to iCloud.com with your Apple ID.

Go to Find iPhone.

Click All Devices at the top of the screen.

Click your Apple Watch.

Click Erase [device]. Select Next until the device is erased.

Click Remove from Account.

Then;

Setup your Watch to a new iPhone