If you have identical devices, even if they are synced to different computers using different iTunes, then this is the way to save the download bandwidth...
You can choose to do the download via iTunes for the first device, or directly by picking the correct ipsw file from https://ipsw.me which simply provides direct download links - the downloads themselves come directly from Apple in both cases.
Assuming you are on Mac...
if you are using ipsw.me then skip this step.
Connect the first device then when asked about the update select "Download Only". This gives you chance to sync & backup first, but otherwise doesn't affect the update. Allow the update to finish, then disconnect the device.
You now have the downloaded file.
If you are going to share this update to other computers, or other Mac user accounts on the same machine, then you need to find it & copy over...
From Finder, open a new window then hit Cmd ⌘ Shift ⇧ G for 'Go To...' then copy paste
~/Library/iTunes/iPhone Software Updates & hit Enter ⌅
Inside there you will see the downloaded ipsw file.
now rejoin if you got the file from ipsw.me... but remember the path from the last step
Copy [don't move] this to your other locations using the same method to find the folder, or if the other users are on the same machine, copy to a 'public' folder that all users can see, then get each user to copy from there to their own folder
[each user has their own folder, accessible from their own account using the same path, as in 3. above].
Connect any of your identical devices to iTunes from the correct user account for that user.
You can now begin the standard update process, the update file is already there & will be recognised.