What happens if you downloaded iOS 6 via iTunes and then after the download was complete you’ve receive an error that updating your iPhone failed?
Luckily there’s a solution to update your iPhone/iPad manually using the iOS 6download file.
Follow the steps below to install the iOS 6 download file manually
Open iTunes and make sure your device is connected
Click on the Device under Devices on the left and the main content screen will open up
Above the Restore button the will be an Update button or Check for Updates button, click on this button holding down the shift key. This will open up a Window where you will be able to select the downloaded file.
In Windows 7 this file will be under c:\Users\YOURUSER\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates
Once the file has been selected the installation of iOS 6 will start.
NOTE: this has been tested on the iPhone 4