You can go to the watch settings on the watch, either by accessing the watch quick panel (seipe down from top of screen), or from the apps panel. In settings go to general, scroll down to reset, tap it and affirm your choice.
Why would you let someone use your watch? It would certainly screw up any health tracking and give them access to your samsung and Google accounts, since you're already logged in. You may have given them the opportunity to remove the watch from your account and add it to theirs, in which case a factory reset won't help you
