Many Watch users are reporting problems getting the watchOS 2.0 update to install, just as I have been.
The most common problem is that the iPhone Watch app stalls, typically when “Verifying…”. The update has been downloaded to the iPhone, and ‘prepared’, but cannot be verified.
Provided that installation has not yet started on the Watch (this is obvious when the Apple logo appears with progress tics around it in a circle, and the Watch app reports ‘Installing’), the best way to break out of this deadlock is to restart both the Watch and the iPhone, probably in that order. To do that, hold the two buttons on the Watch in until the screen goes black, then the white Apple logo appears. Repeat this two-button hold similarly on the iPhone.
Once the Watch then iPhone have been restarted, ensuring the Watch is on its charging stand, open the Watch app on the iPhone and try again. With a bit of luck it will magically verify, and installation will start.
Whatever you do, avoid interrupting anything once installation has started, or you could brick your Watch, requiring a return to Apple.
I wish you success.