Need to hard reset Apple Watch

My step son had an iPhone a while back and we connected my husband’s Apple Watch to it so he could use it. My step son has since gotten rid of the iPhone (uses Samsung now) and we have no record of his iCloud username, password, or phone number. The watch is basically new and unusable because it is paired to my step son’s old phone. How can we do a factory reset on the watch so it unpairs from the previous phone and can then be paired with my husband’s phone? We’ve been trying to resolve this for over a year and all we want is to be able to use the watch again. Thank you!