To update the firmware on your Apple iPhone, download ipsw-distribution of the operating system iOS you need edition. Links to all the firmware posted here:
Please note that each firmware is specific to that model – in other words, the iPhone 2G, 3G, 3Gs and 4 have different ipsw files for the same firmware. Download the distribution for your machine.
After the distribution of the operating system downloaded, launch iTunes on your computer and connect your phone via USB cable. If iTunes is not installed on your computer, you can download it for free from Apple's website:
The Apple iPhone will make the process of synchronizing the iTunes library. Once the sync is complete, go to the tab "Review" in the Apple program iTunes. Hold down the Shift key on the keyboard and at the same time click the Restore button in iTunes. Before you open Windows Explorer. Locate the folder where you saved the firmware file, open the folder and double-click the ipsw file.
After that, the screen will display the extract process and upload (restore) in the form of a filling line. Do not disconnect the iPhone from the computer and don't make a phone any action until Apple iTunes will not inform you about the successful restoration of the iPhone.