How to reset an series 1 apple watch 38mm?

I recently bought an apple watch series 1 38mm from 3rd party. I bought it for my niece for christmas. It is not activation locked but it is saying wrong password that I have to go into the phones app and re enable the passcode. I don't have the phone and I don't know the guy that does. I have try google and have tried the holding the side button till the power off slide bar comes up and hard pressing it but nothing happens when hard pressed so the master reset never comes up. So my question is how else can I master reset this apple watch? I am desperate to get this watch reset so she can use it. Thank you.