Try looking at this thread, find the appropriate firmware / IOS and download it on your computer. When you plug into iTunes, you should be able to do a shift & right click on the device to select the file you downloaded. Maybe this will work for ya. http://forums.everythingicafe.com/threads/ios-official-download-links.76516/
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Scott AKA RoofMonkey
This is why I usually find the newest version of iOS for my particular iPhone and download it first. Then I hold the SHIFT key when I click “Update” and it allows me to navigate to wherever I chose to save that download and use it to update my iOS software.