How Downgrade iPhone 3GS From iOS 6 to iOS 5.1.1 (Windows)
Create a folder on your desktop called Pwnage
Download the latest version of RedSn0w and place it in the Pwnage folder. Likewise, download the firmware you would like to restore to and the firmware you are current on and place it/them in the Pwnage folder.
Extract the RedSn0w zip file to the Pwnage folder.
Right click or control-click the redsn0w application from the redsn0w folder we extracted earlier and choose Run as administrator from the popup menu that appears.
Click Extras from the main menu.
Click Even More from the Extras menu.
Click Restore from the Even More menu.
Click IPSW from the Restore menu, choose the IPSW you would like to restore to from the window that appears, and click Open.
Redsn0w will ask you if you would like to prevent the baseband from being updated. Click Yes.
Redsn0w will inform you that Pwned DFU mode is needed to restore. Click the OK button.
Connect your device, turn it off, and then click the Next button
RedSn0w will now guide you through the steps to get into DFU mode.
Hold down both the Home button and the Power button for 10 seconds.
Release the Power button and continue holding the Home button until RedSn0w detects the device.
Your iPhone will now be exploited with limera1n.
RedSn0w will check Cydia to see if you have the correct blobs saved.
If RedSn0w is unable to find your blobs automatically you can try to click the Remote button to try again or you can click the Local button to search for blobs on your computer.
RedSn0w will now prepare a system restore image.
Your device will then be restored to the new firmware.
You'll be informed when your restore has completed successfully.