The following article will show you how to perform a safe downgrade to 6.15.00 baseband on any iPhone 3GS/3G. We will use Redsn0w 0.9.14b1, the new solution for downgrade the iPhone 3GS/3G baseband 06.15.00 to 05.13.04, which can be unlocked with Ultrasn0w.
However, if you have an iPhone 3GS manufactured after the week 34 of 2011, this downgrade method will NOT work. Specifically, if you have an iPhone 3GS terminal with the serial number xx134 or higher (Settings > General > About > Serial Number) the installation of 05.13.04 baseband is not possible and will result in a total damage of your iPhone 3GS.
Here’s how to downgrade iPhone 3G/3GS
Step 1: Download the latest Redsn0w version and iOS version that you have currently installed on your iPhone 3G or iPhone 3GS;
Step 2: Open Redsn0w downloaded earlier (on Windows right-click on the executable and choose Run As Administrator) and connect your iPhone 3G/3GS to computer;
Step 3: Press the Extras button then click Select IPSW;
Step 4: Navigate to the location where you downloaded the iOS version, select the file and choose Open;
Step 5: You will be asked what iPhone 3GS version you own, namely if it is an iPhone with old or new bootrom;
Step 6: Now press the Back button and then click Jailbreak;
Step 7: Check the option Downgrade from baseband iPad. Uncheck the option Cydia if you already have jailbreak on your iPhone;
Step 8: Close your terminal completely using the Lock button at the top and the slide closure. Next enter your iPhone in DFU Mode;
Step 9: Now will start the downgrade process;
Step 10: The downgrade process will take for several minutes (5 to 8 min.), on the screen will appear the pineapple logo and the text Flashing Baseband. When completed, your iPhone will automatically restart;
Step 11: After the reboot, you should now have an iPhone 3G/3GS with 5.13.04 baseband and ready for the unlock process;