Force restart iPhone – Apple Support
If this does not work, unpairing the watch would be the next step:
Force restart iPhone – Apple Support
If this does not work, unpairing the watch would be the next step:
1 . Identify the model of Apple Watch
2 . Make sure that the previous owner removed Activation Lock from the watch and that the watch has been erased. They must unpair the watch and remove Activation Lock by entering their Apple Account password, so you can use the watch.
3 . For cellular models of Apple Watch, make sure that the previous owner removed and cancelled any active cellular plans.
Try these steps:
If the update won’t start:
You restore your new iPhone from the backup of the old device and pair the watch after that, restoring from the last watch backup, as explained in this article?
Pair your Apple Watch with a new iPhone – Apple Support
Also have a look at this article:
If you can’t pair an Apple Watch with your iPhone – Apple Support