Here’s how to downgrade your iPhone baseband from 04.12.01 to 04.11.08 or 04.10.01 or 01.59.00 if you upgraded to a newer iOS version and lost your jailbreak/unlock.
A friend asked me a few days back if I knew how to downgrade an iPhone 4 baseband from 04.12.01. He upgraded his AT&T iPhone to iOS 5.1 and painfully discovered that there is no way to unlock that baseband, and so he wants a downgrade.
Heads Up: First things first. Make sure you have your SHSH blobs saved locally or on Cydia.
Downgrade Step 1: Frist of all, you need a restore copy of iOS 4.3.5 which you can download here.
Downgrade Step 2: Make sure that your hosts file redirects Apple’s requests to Saurik’s server. Or you can use TinyUmbrella to make iTunes send requests to Cydia’s server by going to the Advanced options.
Downgrade Step 3: Now Turn OFF your device and plug it in to the computer.
Downgrade Step 4: Put your device into DFU mode: Press power button for 2 seconds. Hold down both the Power and Home buttons together for 10 seconds. Release the Power button but keep holding the Home button for 10 more seconds. If your screen is black, you’re in DFU mode. If an iTunes logo is displayed, you’re in Restore Mode: just try the steps above a few more times, sometimes it doesn’t go well on the first try.
Downgrade Step 5: Now manually select firmware from iTunes in restore window. For doing that press Shift and Restore on Windows and Option + Restore on Mac.
Now iTunes will send data to Saurik’s servers instead of Apple’s. Haha.
With help of this tutorial you should successfully downgrade from iOS 5.1 to iOS 4.3.5 and downgrade iPhone 4 baseband 04.12.01 to 04.11.08, 04.10.01 and 01.59.00. Now you’re able to use Gevey SIM unlock again. If you want to use iOS 5 on your iPhone and keep Gevey SIM unlock check our Unlock iPhone 4 using Gevey Sim on iOS 5 guide.