how to reset an apple watch you bought used

The person that you bought/got this watch from needs to follow all the steps on this page;

Remove the Activation Lock so that the buyer can use it.

If they can't or won't then get your money back or recycle the watch and mark it down as a bad experience.

Neither the users on this forum or Apple can help you,only the original owner can help you.

If its activation locked, which it likely is, you cannot reset it. Only the owner who locked the watch can remove activation lock. If you don't know the previous owner, or they cannot remove activation lock you bought yourself a totally useless wrist ornament, which cannot be used as a watch. In a situation such as this, either get the previous owner to remove the lock or return the watch for refund. If neither is an option, Apple will NOT unlock the watch, nor can we help you unlock the watch, so recycle it as it's useless.

The All Too Common SAD Reality of Buying … - Apple Community

That's good. Provide the family member the following link --> What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support