Hello, Follow these steps to Erase your Apple Watch,
- On your Apple Watch tap Settings > General > Reset > Erase All Content and Settings.
- For Apple Watch Series 3 and above (GPS + Cellular), choose to keep or remove your cellular plan.
If you want to pair your Apple Watch iPhone again, keep your plan.
if you don't want to pair your Apple Watch and iPhone again, remove your plan.
If you are aren't going to pair with a different watch or iPhone, you might need to contact your carrier to cancel or update your cellular subscription.
Tap Erase All to confirm. This resets your Apple Watch to the factory settings.
Try these steps If you forgot your Apple ID password - Apple Support