New phone same watch how to pair

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

You can start with this, How to pair your Apple Watch with a new iPhone - Apple Support and if you are experiencing a problem, follow the information in the Related Article. If you need additional help, come back and explain where the problem is.