We’ve created a custom firmware 3.1.2 for iPhone 3GS file on MAC with the help of Pwnage Tool, which will not upgrade your baseband, still you can use ultrasn0w for network unlock.
All you need to download the custom firmware 3.1.2 for iPhone 3GS from here (Please, let us know if you face any problem in downloading this file, we’ll post new links).
Once the firmware file is downloaded just follow these steps to update to OS 3.1.2:
First get your phone to the DFU mode (see how to enter DFU mode here) first and then follow the instructions.
- Make sure you have latest iTunes (9.0.1)
- Launch iTunes
- Connect your iPhone to your Computer
- When your iPhone gets detected by iTunes in DFU mode, go to iPhone, go to Summary tab.
- Hold the Shift button from your keyboard and Press the RESTORE button in the Summary tab.
- A browse window will open, Locate the Custom Firmware 3.1.2 file that you have downloaded earlier.
Here, you go, wait for it to reboot.
Note: This custom firmware file contains Cydia, all you need to do is to launch cydia and install ultrasn0w to achieve network unlock.