Open the Settings app on your Apple Watch
Tap General
Scroll down and tap Reset
Tap Erase All Content and Settings
If prompted, enter your passcode
If prompted, enter your Apple ID password to turn off Activation Lock
Keep the watch on its charger during the reset
Wait for the reset to complete and for the watch to restart
To set up again, follow the on-screen setup steps on the watch
If you can’t access the watch, use your iPhone: open the Watch app > General > Reset > Erase Apple Watch Content and Settings
If you still can’t reset it, use a computer: connect the Apple Watch to your computer, open the Finder (macOS) or iTunes (Windows), select the watch, and choose the option to restore the watch to factory settings (if available)