On your Apple Watch, open the Settings app
Tap General
Tap Reset
Tap Erase All Content and Settings
Enter your passcode if prompted
If asked, choose to keep or remove your cellular plan
Confirm the erase
Wait for the Apple Watch to reset and restart
If you do not have the watch with you, open the Watch app on iPhone
Tap My Watch
Tap General
Tap Reset
Tap Erase Apple Watch Content and Settings
Confirm the erase