How To Downgrade iPhone 4 iOS 5.1 Baseband 4.12.01 to 4.11.08 And Wait For Unlock [Guide]

Apple’s major iOS 5.1 update which was released on March 7th alongside the new iPad also brought two new basebands. These new basebands are not going to hurt people who have factory unlocked iPhone 4 or iPhone 4S but people who rely on iPhone unlocks can put themselves in a big trouble by updating their iPhone to iOS 5.1. We have already warned people who rely on unlock to stay away from this update but some people don’t know the consequences of this specific iOS update. Though it will effect both iPhone 4 and iPhone 4S owners but people who have unlocked iPhone 4S with Gevey Ultra S, TP SIM or R-SIM could get in to more trouble than iPhone 4 owners. Once you update your iPhone 4S to iOS 5.1 you will not only loose your iOS 5.0.1 untethered jailbreak but you will also loose your iPhone 4S unlock because no solution has been updated to unlock iPhone 4S’ new baseband 2.0.10.





If you have an iPhone 4 (carrier locked) and you’ve upgraded to iOS 5.1 then the damage has already been done. You have updated your iPhone 4′s baseband to 4.12.01 which is not unlockable. You cannot go back to any unlockable modem firmware of iPhone 4 like 1.59.00, 2.10, 3.10 or 4.10 but there’s still a hope for you guys to go back on iOS 5′s baseband 4.11.08, there’s no unlock solution available for baseband 4.11.08 (as of this writing) but  there’s a hope for 4.11.08 unlock because all dev teams and hardware unlock manufacturers are working on an unlock solution. Baseband 4.12.01 on the other hand is a new baseband and developer are not familiar with it right now. So there’s a good chance that you will get your iPhone 4 baseband 4.11.08 before than baseband 4.12.01 unlock.
You can downgrade your iPhone 4′s iOS 5.1 baseband 4.12.01 to 4.11.08 and wait for unlock which is the best option available right now. You must do it quickly because Apple will stop signing iOS 5.0.1 any moment and you won’t be able to downgrade your baseband after that. If you think “something is better than nothing” then get your iPhone 4 back on iOS 5.0.1 (baseband 4.11.08) and wait for an unlock.
Note: The window of baseband downgrade is very short you must do it ASAP.

How To Downgrade iOS 5.1 Baseband 4.12.01 to 4.11.08:

Things you will need during this process: 

• iPhone 4
• iOS 5.0.1 IPSW firmware file (Download)
• iTunes (Download)

Step 1: Connect your iPhone 4 to computer via USB cable and make a back up of your data (recommended)
Step 2: Turn off your iPhone and put it in DFU mode
To enter DFU mode:

• Hold Power and Home buttons for 10 seconds
• Now release the Power button but continue holding the Home button for 10 more seconds
• You device should now be in DFU mode (black screen)

Step 3: Launch iTunes, it will recognize your iOS device in recovery mode. Now we’re going to restore the IPSW file which you downloaded from the link above

• Windows users: Press Shift+Restore
• Mac  OS X users: Press Alt+Restore

Step 4: iTunes will restore your iPhone 4 back to iOS 5.0.1 and your baseband will be downgraded from 4.12.01 to 4.11.08.

This was it folks, though your iPhone 4 will remain locked but you have a better chance of getting it unlocked on baseband 4.11.08.
If you face any problem during this process feel free to bug us!
We will keep you updated on this, stay tuned!

I found that's usfull for those have unfortunatly upgrade their iPhone, and a good warn for those are still in 5.0.1 . stay away to 5.1 before a Dev feedback about it 

Givethank to How to Arena blog