Updating iphone 3g
Apple recommends updating i Phone or i Pod touch to use the latest software.
Through what can only be described as relentless and consistent improvement over the years, Apple has made i OS one of the most feature-rich and well-supported platforms on the market.
This might happen while you trying to Jail Break then something went wrong, and you end up with a i Phone which had nothing but Apple Logo in it. You can't turn it off completely or restart or even no response while you connect it to PC/Mac and no response from i Tune too.
Here I will explain solution to this problem and details about DFU and Recovery Mode.
If you choose to restore, all data is erased from i Phone or i Pod, including songs, videos, contacts, photos, calendar information, and any other data.
All i Phone or i Pod settings are restored to their factory condition. Do not disconnect your i Phone until the update has finished.
Visit Stack Exchange No ,you can't update the the i OS 4.2.1 to i OS 5 in i Phone 3g.