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
STEP 1
Open iTunes and make sure your device is connected
STEP 2
Click on the Device under Devices on the left and the main content screen will open up
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STEP 3**
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
STEP 4
Once the file has been selected the installation of iOS 6 will start.
NOTE: this has been tested on the iPhone 4