On my first hub it said there was NOT an upgrade when I did the "check for updates" . After I rebooted the hub it found the update file.
On my second hub it would freeze during the download and not download the full file. After a reboot it downloaded the update just fine.
On my third hub everything was fine, saw the update, applied it, etc.
My suggestion (which I should have done in the first place, to be honest) is to reboot before checking for/installing the update
(all hubs are on the same subnet and have minimal CPU loading btw) I'm in a rural area and controlling lights on multiple buildings is beyond the range of one zwave network.