Even many people have ditched iPhone 3Gs and bought iPhone 4 or 4s, it seems iPhone Dev Team do not ever forget iPhone 3Gs users. That’s true. Recently they’ve just released newer version of Redsn0w bringing ability to downgrade jailbroken iPhone 3Gs baseband 6.15.00 (not unlockable so far) to 05.13.04 (unlockable with Ultrasn0w). Besides, Redsn0w can also fix several issues regarding 3Gs’ GPS.
Downgrading iPhone 3Gs Baseband With RedSn0w
Once you’ve downloaded (link below) and extracted RedSn0w, simply give it a run but make sure you’ve put your iPhone 3Gs plugged into your PC/Mac. Now click on Extras >> Select IPSW button >> browse for the IPSW file >> turn your iPhone off normally via “slide to power off” >> click Back on Redsn0w >> click Jailbreak >> “Downgrade from iPad baseband” checkbox >> uncheck “Install Cydia” >> click Next >> follow on-screen instruction to put your iPhone into DFU mode >> now let RedSn0w does its job.
That’s it and once your iPhone reboots, you’ll have baseband 05.13.04 on it. It means you can now proceed to install UltraSn0w for unlocking.