Want to update to iOS 4.3.1 but preserve iPhone 3GS baseband 4.3.1? Now you can update and jailbreak iPhone 3GS 4.3.1 and preserve 3GS baseband to be able to unlock iPhone 3GS 4.3.1 with ultrasn0w 1.2. This can be achieved either with PwnageTool 4.3 or with Sn0wbreeze 2.5. We have already posted a PwnageTool 4.3.1 guide for Mac users, here’s Sn0wbreeze 4.3.1 tutorial for Windows users:

Before you Begin:

• Download iOS 4.3.1 firmware for your device
• Download Sn0wbreeze 2.5 for Windows
• Download iTunes 10.2.1

Step 1: Unzip sn0wbreeze-2.5.zip and launch sn0wbreeze-v2.5.exe

Step 2: Click on “Browse” button and select the iOS 4.3.1 firmware IPSW for your device.

When you click next button, Sn0wbreeze identify the IPSW that you browsed for. Once that process is complete, click on Next button. For iPhone 3GS, it will ask if your device is New Bootrom or Old Bootrom.

Step 3: Click on the “Baseband Preservation Mode”.

Step 4: Sn0wbreeze will now start the process to jailbreak iOS 4.3.1.

Step 5: When iOS 4.3.1 process completes, you can find the custom 4.3.1 IPSW on your desktop. You will see following message on Sn0wbreeze:

Get into DFU mode to load custom IPSW that will allow you to preserve baseband while upgrading to iOS 4.3.1. If you need help getting into DFU mode, Sn0wbreeze DFU Pwner will help you do that with on-screen instructions.

Step 6: To start with, switch your device off and connect it to the PC via USB. Then click Start button.

Step 7: Now follow the onscreen instructions to get into DFU mode.

Step 8: Once your device is in DFU mode, open iTunes and select your device from left sidebar.

Step 9: Press and hold Shift key (Windows) or Alt key (Mac) and click on “Restore” button.

Step 10: Browse and select the custom iOS 4.3.1 ipsw that you just created with Sn0wbreeze.

That’s it, enjoy!