I purchased an apple watch that is still linked to a previous owner. How do i unlink it to allow me use on my device?

The person that you bought this watch from needs to follow all the steps in this link;

Prepare the watch so the buyer can use it.

If they can't or won't then get your money back or recycle the watch and put it down to experience.

You can't the previous owner needed to have done that before handing to you .