How often has it happened that iTunes is busy downloading the update file for your iOS device, and it fails, with NO RESUME!! So you have to start all over.
This page might help, because you get a direct link to the file on the Apple site, and you can use a download manager to get the file with resuming.
Thanks to OSXDaily
Firmware files have an .ipsw extension and can be found at the following locations:
On Windows XP :
Documents and Settings
On Windows Vista/Windows 7:
Users[UserName]
On Mac:
~/Library/iTunes/iPhone Software Updates
Thanks to iPhone FAQ
You can then place the file in that location and run the update, iTunes will pick the file up, verify it and skip the download portion of the update.