Manually Update to iOS 6
The directions are going to be the same for Windows and Mac OS X users with the exception of where the files are stored:
Using Chrome, Firefox, or Safari, download iOS 6 IPSW for your device, and choose “Save As” when you download the file, saving it somewhere easy to find like the desktop
Quit iTunes
Disconnect your iOS device from the computer
Copy the previously downloaded IPSW file to one of the following locations, depending on your desktop OS:
For Windows
Go to the Start menu, choose Computer, Local disk, enter the following path:
c:\Users\NAME\AppData\Roaming\Apple Computer\iTunes\
Now you are looking for a directory like “iPhone Software Updates” – this is device dependent so it could be “iPad Software Updates” or “iPod Software Updates” on your PC
Delete existing iOS 6 .ipsw files from this folder and copy over the version you downloaded
For Mac OS X
From the Mac desktop, hit Command+Shift+G and type the following path:
~/Library/iTunes/
If you’re updating an iPhone, the folder will be named “iPhone Software Updates”, iPad will be “iPad Software Updates” and so on, open this folder
Move the previously downloaded iOS 6 IPSW file into this folder
For Everyone
Now relaunch iTunes, choose your device from the left side, and click on “Check for Update” to use the new IPSW without re-downloading
This should work without any of the unknown errors because the file no longer has to be downloaded from Apple’s servers. Much of the trouble probably relates to user hosts files or firewalls, but this has the added benefit of the manual hunt for the firmware file which is another place where some confusion was caused. Enjoy iOS 6, it’s the best iOS yet.