The iPhone is a smart phone developed by Apple that runs the iPhone operating system (iOS). As of November 2010, iOS 4 is the current version of the operating system. The operating system on older iPhone models including the 3G and 3GS can be upgraded to iOS 4 through the iTunes interface, but some users may find that the updated version of iOS runs slowly on their older iPhone models. Reversing a previous iPhone update is not an easy process, but it can be done with the right utilities and recovery commands.
Locate the "iPhone Software Updates" folder on your computer because it will contain the previous version of the iOS operating system firmware. On Windows, you can find the folder by typing "%appdata%\Apple Computer\iTunes\iPhone Software Updates" into the Start menu search box and then pressing "Enter." On Mac, the folder is located at "~/Library/iTunes/iPhone Software Updates." If you do not see previous versions of iOS, you can download the version you wish to install from iClarified. (See Resources.)
Connect the iPhone to your computer.
Turn off your iPhone and wait for at least five seconds before proceeding to the next step.
Press the "Power" button and the "Home" button on your iPhone at the same time and hold for 10 seconds. After 10 seconds, release the "Power" button but continue to hold the "Home" button. This will put your iPhone into recovery mode.
Launch iTunes. The program will detect that your iPhone is in recovery mode and tell you to restore the device.
Select "iPhone" from the "Devices" list in the iTunes sidebar.
Hold down the "Shift" key and then click the "Restore" button located at the bottom-right to select the iOS file you wish to use.
Select the iOS file you wish to downgrade to from the folder you located in Step 1. These files have a .ipsw file extension. The installation process will take 10 minutes and you may receive an error when it is completed, but this error can be disregarded.
Launch a Web browser and download the RecBoot utility to kick your iPhone out of recovery mode. (See Resources.)
Launch RecBoot and click "Exit Recovery Mode." Your iPhone is now downgraded to a prior version of the iOS and you can now restore iPhone backups using iTunes.