Apple Watch 7 unable to update to OS26.6-now useless

If the watch is completely frozen or stuck, standard resets through the iPhone app often fail. 

Never do this when updating watchOS!

1. Press and hold the side button on your Apple Watch until the power-off icon appears in the top right corner. 
2. Press and hold the Digital Crown down until the screen changes to "Erase All Content and Settings".
3. Tap Reset, then tap Reset again to confirm. 

There have been a number of other threads on this matter recently, mostly involving the Ultra 1. Based on what people are saying that Apple Support told them, it is not possible to pair a watch with iOS 26.x to a phone with iOS 27.x.