How do I unpair my watch from a phone I no longer have? I need to pair it with my new iPhone 13

See the directions in this link, paying attention to the section "If you don't have your old phone." --> How to pair your Apple Watch with a new iPhone - Apple Support

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.