(There’s no specific forum for iPadOS… so I guess this currently remains the best fit.)
I *always* recommend updating iOS/iPadOS devices via iTunes rather than OTA (Over The Air via Wi-Fi). Even when using the very clunky Windows version of iTunes (and *never* via the MS Store version of iTunes… too many tears).
This is primarily because using iTunes on a PC/Mac allows the update to download and expand on the PC/Mac rather than on the iOS/iPadOS device itself… which often has too little space left for the unpacking.
So… I tried so many times to update my iPad (Gen 6) to iPadOS 14.0.1 when it became available (as shown by a notification icon). Yet my iPad has loads of free space:
Even though I updated as advised to the latest version of iTunes (12.10.9.3 currently), the iPasOS update failed consistently within iTunes at ‘Step 2 of 5’. I swapped out all my (genuine Apple) USB-to-Lightning cables in turn, all to no avail. On my iPad it just showed that an error occurred *every* time:
Within iTunes it showed an ‘Error 4000’ occurred *every* single time.
I tried every single one of the potential solutions recommended automatically by the failed installation (updating, rebooting, cable swapping, weird incantations… OK, perhaps not the last)… and by my own Googling (this is now a verb, right?) about the specific error code (for example).
Finally I got the update to install… by using an OTA update via Wi-Fi and I’m still at a loss to explain why a Wi-Fi OTA update worked when a direct USB-to-Lightning cable connection didn’t.
Hope this helps others if they run into the same thing…