Today, the iPhone Dev Team has released an updated version of redsn0w, which includes a baseband downgrade option, for those who are using the iPad’s 06.15 baseband on the iPhone 3G or iPhone 3GS.
Typically you’d have the 06.15 baseband if you unlock with ultrasn0w but updated your iPhone baseband past 05.13.04. With this new capability, you can now downgrade specifically from 06.15 to 05.13.04 (even if you never had 05.13.04 on that device before). This gives you the best of both worlds: ultrasn0w compatibility and a normal iPhone baseband with full GPS and the ability to use stock IPSWs again.
NOTE: the following tutorial shows how to use redsn0w to downgrade your baseband on the Mac. On Windows, the process is identical with one small difference: you have to run redsn0w as Administrator .
1. Download redsn0w v0.9.14b1
2. Download the current firmware version of your device ( download here )
3. Run redsn0w
4. Click on ‘Extras’
5. Click ‘Select IPSW’ , browse and select the firmware (.ipsw) that you have downloaded earlier and click open
6. Click OK to confirm your selection
7. redsn0w will ask you if you have an older or newer model of iPhone 3GS ( just for iPhone 3GS )
8. Now click on the ‘Back’ button to get back to the main menu and click the ‘Jailbreak’ button
9. Wait a few seconds for redsn0w to prepare the jailbreak data. Now check ‘Downgrade from iPad baseband’ , uncheck Cydia if you already have it installed and click Next…
10. Plug-in your iPhone and turn it off. Once the iPhone is off, click Next
11. Put your iPhone in DFU mode. redsn0w will guide you through the process if you never did it before
12. From now on, everything is automated. Redsn0w will start some automated processes that will reboot your device. Once redsn0w is done, you will see your iPhone going into verbose mode ( scrolling text on your screen ) followed by a picture of the pwnapple icon which will say ‘Flashing Baseband ( don’t interrupt! )’ . This process takes about 10 minutes or so, and you won’t be getting any feedback during that time. Don’t do anything. Just wait.
13. Once your iPhone reboots, you’ll have the 05.13.04 baseband. Now load Cydia, and install ultrasn0w to unlock. If you don’t have it add the following repo into Cydia http://repo666.ultrasn0w.com