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 reset
Wait for the Apple Watch to erase and restart
To reset using your iPhone, open the Watch app
Tap My Watch
Tap General
Tap Reset
Tap Erase Apple Watch Content and Settings
Confirm the reset
If your Apple Watch is unresponsive, place it on its charger
Press and hold the side button until the power off slider appears
Press and hold the Digital Crown until the erase option appears, if available
Follow the on-screen prompts to reset it