Hi
Try these steps, which may help to address issues at or before the "verifying" stage. However, do not interrupt your iPhone or watch if the process has already reached the "installing" stage.
On your iPhone, close the Watch app along with all other open apps (double-press on the Home button, swipe up on each of the app previews).
Remove your watch from the charger and restart both devices, turning both off together first and then restarting your iPhone first.
- To turn your watch off: press and hold the side button until you see the Power Off slider; drag it to turn off.
- To turn it back on: press and hold the side button until you see the Apple logo.
Then replace your watch on the charger and try updating again by following the instructions here - and also keeping your iPhone connected to power:
Update the software on your Apple Watch - Apple Support