1. Double-click on the iTunes application icon to launch it.

2. Plug the sync cable into the "Sync" port at the bottom of the iPhone, then the other end of the cable into an available USB port on your computer.

3. Press the "Power" button to turn the phone off, then hold the "Power" and "Home" button down for 10 seconds. Release the "Power" button but continue to hold the "Home" button until the computer beeps. The iTunes application will detect the phone. This will place the phone into DFU mode. A pop-up window will appear saying, "iTunes has detected an iPhone in recovery mode..." Click on "OK."

4. Hold the "Option" key while clicking the "Restore" button in the iTunes application, then scroll through the firmware listing that appears. Select "OS 3.1.2," then click on "OK." The downgrade should take about 10 to 15 minutes to complete.