Lost iphone and need to unpair watch

Hi

In order to be able to pair your Apple Watch with your replacement iPhone, you will first need to erase your watch manually:

• On your Apple Watch, go to: Settings > General > Reset > Erase All Content and Settings.
• Unpair your Apple Watch and iPhone - Apple Support

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

If you have not yet done so, also be sure to follow the instructions here in respect of your lost iPhone:

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