My download of Apple watchOS 10.4 STOPPED during "preparing" and won't advance.

See the 'Need more help with updating?' section in:

Update your Apple Watch - Apple Support

Update was competed at about hour 2 after restarts of watch and iPhone. It might have completed on prior try if I had waited, but it was taking so long with no indication of progress.

iPhone XR, like yours, Apple Watch 8

down load of apple watch 10.4 is stuck and does not advance in preparing stage

Same here with my MR983QL/A (series 9)

Same

How do you delete the update?

I restart both of the devices (iPhone and Apple Watch) and start the update.

i let it during the night and today it was done👍🏻

Under airplane mode I selected mirror iPhone and the update progressed quickly and installed.

I have an iPhone 15 pro

watch series 9

Iphone 11pro update was ok but watch os stopped on my Apple Watch 6 during preparation half way

Did u found any solution?

I had this issue and a reboot of watch and phone resolved it.

You should also try deleted 10.4 in storage on watch before it installs. then try again.

Yes. You are right. Needed to take longer time to download especially during the phone in “preparing “ status. Just leave it for 1.5 hours and it will automatically install the watch is 10.4.

@tomac14, not sure I agree with you here. My iPhone 11 has latest IOS, and watch was on charger all night (7 hours), the next morning still showing circa 60% on prep bar and update not installed.

Frankly, the “leave the room” is where you lost me. BTW…this didn’t work.