A Google search of "how does one unpair an Apple watch from an iPhone that doesn't work" gave me this:
To unpair an Apple Watch from an iPhone that doesn't work, you can erase the Apple Watch:
- Open the Settings app on the Apple Watch
- Tap General > Reset
- Tap Erase All Content and Settings
- Tap again to confirm
- Wait for the watch to unpair
Erasing the Apple Watch will reset it to factory settings. However, Activation Lock will remain, so you'll need to enter the Apple Account and password used to set up the watch to pair it again.
If you know your Apple Watch passcode, you can also unpair it by going to Settings > General > Reset.
There were other results so you should investigate them also, like How To Unpair Apple Watch without iPhone
