To Everyone who is having trouble with the download of Watch OS 10.4:
Apple Support states:
"Updating to watchOS 10 requires iPhone XS or later with iOS 17."
This is WRONG.
You can use several iPhone series to download Watch OS 10.4, not just iPhone Series XS.
I did it with iPhone Series XR. Others did it with Series 8 and other series.
Just RESTART (Turn OFF and then ON) your iPhone and Apple Watch.
Then use your iPhone to start the download, then put your watch on its charger.
It takes a LONG time and will APPEAR to get stuck on "Preparing the download"
Just leave it alone and let it finish the "Preparing".
Leave the room where the watch is being updated. Do NOT watch it.
It takes about 1.5 to 2 hours.
During this time the sliding horizontal bar on the iPhone does NOT MOVE.
Come back after 2 hours and it will be updated. If the watch face is dark, just touch it.
Hope this helps everyone with this problem.
Tom