- 1). Download the iOS 3.1.3 firmware file using your computer. Apple offers free download links for all previous iOS firmware files (see Resources).
- 2). Double-click the iTunes icon on your computer.
- 3). Connect your iPhone to your computer using the phone's data cable.
- 4). Select your iPhone from the "Devices" subsection in iTunes.
- 5). Press and hold the "Shift" key on your computer.
- 6). Click the "Restore" button at the bottom of iTunes.
- 7). Select the downloaded iOS firmware file.
- 8). Click the "Confirm" button. ITunes downgrades the iOS firmware file on your iPhone and prompts you when the downgrade completes.
Source...