How to remove someone else’s email off of my Apple Watch that I just purchased

I need help removing someone else’s email address off of the Apple Watch that I just purchased that he just previously owned that he couldn’t afford no more so I went to rent a hometown where you can rent to buy things so I purchased this Apple Watch. I went to go and set it up, but his email address is still on the Watch so I can’t continue to set it up until his email address is taken off