Hi
Follow these steps to erase your locked watch and re-pair it with your iPhone:
- Connect your watch to its charging cable, with power connected.
- Press and hold the side button until you see the Power Off slider.
- Press firmly on the display (do not try to slide the Power Off slider - just press down more firmly than a normal tap) and then tap on Erase all content and settings.
- Re-pair your watch using the Watch app on your iPhone, choosing to Restore from Backup if offered the option.
More information:
Set up your Apple Watch - Apple Support
If you forgot the passcode for your Apple Watch - Apple Support