Unpair my Apple Watch from a stolen phone

Hi

Follow these steps to erase your Apple Watch manually:

On your Apple Watch, go to: Settings > General > Reset > Erase All Content and Settings.

Then pair it with your new iPhone by following the instructions here, starting by updating your iPhone to the latest version of iOS:

Set up your Apple Watch - Apple Support

Be sure to follow Apple's instructions here in respect of your stolen iPhone:

• If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support
• Report a lost or stolen Apple product - Apple Support

See the second section of this article for instructions about how to erase your watch without it's previous parent iPhone.

Unpair your Apple Watch and iPhone - Apple Support