Q: Need to unpair watch from phone but watch is unavailable. Is this possible?

was considering buying an Apple Watch from an individual, paired the watch to my husband's phone when we were trying it out. Did not buy the watch, bought a new one instead, but did not not unpair the first watch. Now we cannot get the watch unpaired or cleared off of my husbands phone so that we can pair the new watch. How do we unpair a watch from a phone when the watch is not available?