How to download any iOS manually?
I've had the same issue, when I had connectivity issues - you're right, it never does appear to resume.

You can get the update file directly from https://ipsw.me
this site merely links to the correct location at Apple for the file you require &amp; so is safe to use  

Get the latest version for your device.
This is not a way to downgrade your iOS, merely to fetch the current one by an alternative method.  

If you still have difficulty resuming, then use a separate downloader app, SpeedDownload for Mac or Wget for Windows, etc.

Launch iTunes.
Plug the phone to your computer.  

Then, in iTunes, hold Opt [alt] when clicking the Check for Update button &amp; a file-picker will open instead. Find the file you just downloaded &amp; it will run from there as normal.