If there is ever a firmware failure on your Apple Watch, the only way to get it fixed would be to send it to an Apple store. This is because only the Apple store or authorized repair centers will be able to properly access the smartwatch’s data port, something that regular users aren’t able to do.
This means that restoring firmware on an Apple Watch isn’t as simple as the iPhone or iPad. However, there is some good news because in the latest iOS 15.4 update, it appears that Apple will now allow iPhone users to restore the firmware of the Apple Watch, saving users a trip to the store and time in the process.
This will require that users have the latest updates on their iPhone and Apple Watch, which at this time of writing is iOS 15.4 and watchOS 8.5, so as long as you have both of those updates, you should be able to restore your Apple Watch’s firmware through the iPhone without any issues.
If you’re interested in knowing how to do it or maybe if you want to bookmark it for later, Apple has published a support document on its website that you can refer to for additional details and troubleshooting.