How to unpair Iwatch without old phone

if to be able to pair with a new one go to How to pair your Apple Watch with a new iPhone - Apple Support

scroll to section "If you don't have your old iPhone or you erased it"

otherwise try reading

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - 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.