Apple Watch will not update.
Try the steps here to troubleshoot not being able to update: Update your Apple Watch - Apple Support
Need more help with updating?
Try these steps:
Make sure that your Apple Watch is connecting properly to its charger.
If your Apple Watch won't charge or won't turn on – Apple Support (UK)
Restart your Apple Watch – Apple Support (UK)
Restart your iPhone – Apple Support (UK)
Try again to update your Apple Watch.
If the update won't start:
Open the Apple Watch app on your iPhone.
Tap General > Software Update.
Delete the update file.
Try again to update your Apple Watch.
if your Apple Watch says “you need more available storage to install a watchOS update.” If your Apple Watch storage is full – Apple Support (UK)