A gray area until recently declared legal, the act of jailbreaking iPhones and iPod touches is becoming increasingly common. Reasons for jailbreaking include running simultaneous applications, downloading more apps and, for the iPhone, unlocking for multiple carriers. Apps like Cydia, the source for most jailbroken apps, do not work with recently released iPhone operating software 3.1.3 because it has yet to be cracked. Many users who mistakenly updated their operating firmware are looking for ways to downgrade back to 3.1.2.

Instructions

1

Open "My Computer" and locate your hard drive. Navigate to "C:/Users/[Your Name]/AppData/Roaming/Apple Computer/iTunes/iPod Software Updates." Look for a file ending with ".ipsw" containing "3.1.2" separated by underscores. This is your old software. For example: "iPhone1,2_3.1.2_7D11_Restore.ipsw." If you can not locate the file, download it from Felix Bruns' website.
2

Disconnect your device if it is connected to the computer.
3

Turn off the device. While it is shutting down, hold the home button for 10 seconds while connecting the device to the computer via USB. If your device displays the message "Connect to iTunes," you have done this step correctly, and the phone is in recovery mode.
4

Open iTunes and click on your device on the iTunes sidebar. When the device page loads, shift+click the "Restore" button. You will be prompted to select the location of the 3.1.2 IPSW file, which you know from locating or saving the file in Step 1. Wait until the installation completes, and be careful not to update again if prompted.
Tips & Warnings

When downloading the file, if you need it, your browser may change the ipsw file to a zip file. In this case, right click the file name and rename the extension from .zip to .ipsw for iTunes to accept it.