Open the Settings app on your Apple Watch
Tap General
Scroll down and tap Reset
Choose one option:
Reset All Settings
Erase All Content and Settings
If prompted, enter your passcode
If prompted, confirm the reset
Wait for the watch to restart and finish erasing
To reset using the watch buttons (when the watch isn’t responding):
Press and hold the Side button until the power off screen appears
Press and hold the Digital Crown
Keep holding until you see Erase All Content and Settings
Release the buttons and follow the on-screen prompts
Confirm the reset if prompted
To remove the watch from your iPhone before resetting (optional):
Open the Watch app on your iPhone
Tap All Watches
Tap the info button (i) next to your watch
Tap Unpair Apple Watch
Confirm Unpair
Follow prompts to erase the watch