iPad not update to iOS 4.2 with Error 1645

iPad not update to iOS 4.2 with Error 1645 - After downloading the iOs 4.2.1 software, I tried to update my iPad to 4.2.1. But in the actual it did not update. The iPad rebooted and then got an error message "1645 unknown error". The download bar maked no progress.

How to fix this issue
* Power the iPad off.
* Connect the USB cable from computer to iPad. Press and hold the Home button. When you see the "Connect to iTunes" screen, release the Home button.
* Open iTunes to update iPad.