How can I reset an apple watch if it’s been given to me after being set up by another user

Activation Lock is designed to prevent anyone else from using an Apple Watch if it's ever lost or stolen.

About Activation Lock on your Apple Watch - Apple Support

Activation Lock is an anti-theft feature that cannot be bypassed. Only the previous owner can remove the lock by using their Apple ID & password. Apple will not help you find the previous owner. If the previous owner cannot or will not remove the lock, then the watch is useless to you or anyone else. Try to get a refund (through eBay) or dispose of the watch properly. In the future, see the 'Before you buy a watch from a third-party' section in:

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

See this link and click on the link if you don't have your old iPhone --> How to pair your Apple Watch with a new iPhone - Apple Support

That will remove the watch from his account and prepare the watch to be pairable to a new iPhone.

You can't. Whoever gave you the watch MUST remove the watch from their account. Assuming they can do that, you can use the watch. Assuming they can't do that, the watch is useless to you.

Well, you can't do it. ONLY the person who owned the watch and locked the watch can do it. Do you actually know the original owner of the watch who set it up?