Downgrade Iphone 3gs 3.1.3 To 3.1.2 With SHSH On Cydia For Windows

1. Download and unzip quickpwn22 on desktop, this application can be used to supply official carrier for you and of course take you out of recovery
2. Download iPhone OS 3.1.2 ipsw
3. Go to iTune and plug-in your iPhone and locate it in DFU mode (remember don't put it in recovery mode)
4. Then hold sleep and home for 10 seconds, and hold home for another 10 seconds till iTunes pop up appears to bring a message that you are in recovery mode now and restore is required to continue
5. In the same time hold shift and click on restore to get you to custom 3.1.2 ipsw file and then restore again when the process done
6. Now an error message or warning: 1015 or 1014 will appear, don't be panic, this means you are doing right! Because you have been backed up on cydia and it's communicating. then you will be in recovery mode where you will see disk and connection cord on screen, just click shift and restore and insert custom ipsw file again hit restore.
7. Another error message 1015 appears at this phase, you need to open quickpwn22 while your still jacked in on itunes, it will give you a message that says hold sleep and home for 15 second then let go of home and continue to hold sleep for 2 seconds, this will get you out of recovery and your official carrier back, next run latest rc3 blackra1n jailbreaking tool

Now you iPhone OS 3.1.3 is downgraded to 3.1.2 with firmware 5.12.01 and official carrier!

Source: Downgrade Iphone 3gs 3.1.3 To 3.1.2 With SHSH On Cydia For Windows