Pair watch

See the "How to erase your Apple Watch if you don't have your iPhone" section in :

Unpair and erase your Apple Watch - Apple Support

Hi

Not sure if i understand the problem clearly, however if you are having the problem that "you have a watch which was paired with a phone which is no more available with you but you now want to pair it with new phone" - if that is so?

You can use the watch to directly reset it -

Reset without iPhone

• Head to Settings on your Apple Watch
• Tap General
• Swipe down and tap Reset
• Tap Erase all content and settings
• If you are using a passcode, enter it when prompted
• Swipe down and choose Erase all or Erase all and keep plan if you have a cell watch

Reset Apple Watch without your password: You can also reset your Apple Watch if you don’t remember your password, follow these steps:

• Put the Apple Watch on its charger
• Press and hold the side button as if to turn it off
• Firmly press the power off slider and release it
• Tap Erase all content and settings.

Start here;

How to pair your Apple Watch with a new iPhone – Apple Support

None of this removes the activation lock from the old phone that the op needs to do.