Open the Settings app on your Apple Watch
Tap General
Tap Reset
Tap Erase All Content and Settings
Enter your passcode if prompted
Confirm the reset
Wait for the Apple Watch to erase and restart
If you do not have access to the watch, open the Watch app on your iPhone
Tap My Watch
Tap General
Tap Reset
Tap Erase Apple Watch Content and Settings
Confirm the reset
If the watch is unresponsive, press and hold the side button and Digital Crown together
Keep holding until the Apple logo appears
Try the reset again after it restarts