This is the guide for those people who are still on old baseband and want to preserve their baseband and update to iOS 4.3.1. If you are on iOS 4.3.1 and want to perform an Untethered Jailbreak so use Redsn0w guide available here.
Things required to perform the following steps are:
1. iTunesLatest Version
2. iOS 4.3.1
3. PWNAGE Tool 4.3
4. An Internet Connection
Below are the Steps to be followed.
Download PWNAGE Tool 4.3.dmg
Once Downloaded Open PWNAGE Tool
When the PWNAGE Tool will be opened, click EXPERT MODE from the Top and from below choose your Device.
When you will proceed next, it will ask for you to browse for the respected firmware which is iOS 4.3.1.
Now select your firmware
After you have selected your Firmware now proceed next.
When you will proceed next it will give you options like this:
Select General and Proceed next
When you proceed next it will give you a screen like this:
When prompted to this window select Build and Proceed next
Now click next and the custom firmware cooking will start.
After some time it will ask you for the admin Password. Enter your Password and press OK to continue.
Once done. It will show you a message DONE!. Then proceed next to put your Device into DFU Mode via PWNAGE Tool
The Custom Firmware will be on your Desktop, open iTunes and restore to that Custom Firmware. In no time you will be done and will be on iOS 4.3.1 with the old baseband.
If you rely on Unlock so Follow this tutorial here to Fix Ultrasn0w. It is broken on iOS 4.3.1.