Apple watch not updating to WatchOS 26.6

"Exactly what worked for me, in case it helps: Apple Watch Ultra (1st gen) and an iPhone 17 Pro Max on iOS 27.0.1. I tried multiple restarts and even erased the watch twice, but it kept failing with 'Unable to Install Update.' What finally worked was temporarily pairing it to an iPhone still on iOS 26, choosing to continue setup without updating, then on that phone going into the Watch app > General > Storage and deleting the downloaded update. After that I erased the watch properly and paired it again with my iPhone on iOS 27, and it completed normally. As of today, Apple still hasn't published a clear fix on their support site, and customer service basically tells you to drop off the watch for repair. This looks like a software bug affecting iOS 26 devices that aren't compatible with iOS 27. Apple really needs to publish guidance so users know what's going on."

Check free space storage on your watch. The update may require ~ 1.5 GB and it is not a good idea to go right down to filling the watch completely (or close to completely).

What about an Apple Watch that has been reset to factory settings and needs to be paired again?

I have an older Apple Watch SE (2nd generation) that has not been used for about four years by my father. I unpaired it from his iPhone and reset it to factory settings before trying to pair it with another iPhone to give it to my sister.

I have tried pairing it with an iPhone 14 Pro, 16 Pro, and 17 Pro, all running iOS 26, but I still have the same problem. I can't access the storage on the Apple Watch because it has already been reset to factory settings. I also can't delete or reinstall the corrupted watchOS, assuming that the problem is actually caused by watchOS.

We even went to the Genius Bar in an Apple Store, where they tried to help us for several hours. Unfortunately, the only answer we eventually got was that the Apple Watch is too old and that we should replace it with a new one.

Will there be an update to fix this issue?

Nah hudreds if not thousands of people are facing this issue since iOS 27 update. If this isn't fixed by next update I'll have to throw my watch in the trash and my phone will follow soon as I upgrade to an android.

"Exactly what worked for me, in case it helps: Apple Watch Ultra (1st gen) and an iPhone 17 Pro Max on iOS 27.0.1. I tried multiple restarts and even erased the watch twice, but it kept failing with 'Unable to Install Update.' What finally worked was temporarily pairing it to an iPhone still on iOS 26, choosing to continue setup without updating, then on that phone going into the Watch app > General > Storage and deleting the downloaded update. After that I erased the watch properly and paired it again with my iPhone on iOS 27, and it completed normally. As of today, Apple still hasn't published a clear fix on their support site, and customer service basically tells you to drop off the watch for repair. This looks like a software bug affecting iOS 26 devices that aren't compatible with iOS 27. Apple really needs to publish guidance so users know what's going on."