How to uninstall watchOS beta software?

You cannot get back to OS 9 without sending it in to Apple.

Apple store and authorised repair shops cannot do this work.

It was in the developer notes.

All you can do is continue using the beta until watchOS 10 comes out, and then you can switch to the official release.

If your Apple Watch running the beta will not pair to your iPhone, then you will have to enable the iPhone beta until the iPhone iOS 17 is released.

watchOS 10 and iOS 17 should be officially released this Fall (I am guessing it will most likely happen in the September/October timeframe).