I bought a apple watch from a pawn shop but the iCloud is locked. How do I get it unlocked?

Turn off Find My iPhone Activation Lock - Apple Support

You cannot unlock it. Take it back to the pawn shop and get your money back. You will never be able to use it. Only the person who put an Activation Lock on a device (or someone who knows that person's Apple ID and password) can remove it. There is no way around this.

Best,

GB

You don't. You take it back to the pawn shop and see if you can get your money back.