The iOS 5.1 has been dropped already, there are lots of the reason for this is that some customers would wish to return to a earlier iOS model, specifically if their iPhone unlocks depend on it. You cannot save SHSH blobs once Apple has stopped signing the firmware. If you have already saved SHSH Blobs, follow the tutorial below to downgrade from iOS 5.1 to 5.0.1, 5.0, 4.3.5, 4.3.3
For downgrading to iOS 5.0.1/5.0 custom firmware using iFaith
This guide wont work on iPhone 4S and iPad 2. Only iPhone 4, 3GS, iPad 1, and iPod touch users can downgrade to iOS 5.x custom firmware using this method.
Step 1: Simply download the latest version of iFaith for Windows and the official iOS 5.0.1 firmware.
Step 2: Create a signed firmware file by clicking on ‘Build *signed* IPSW w/Blobs’.
Step 3: Browse for the SHSH blobs file that you previously saved for your current device to downgrade back to the old 5.0.1/5.0 firmware. Next, iFaith will ask you to select the stock firmware of which you want it to create custom firmware file from. Once done, iFaith will have a new custom firmware file for you waiting on the desktop with SHSH blobs incorporated for the firmware you are downgrading to.
Step 4: Enter DFU mode using iFaith and then simply use iTunes to restore to this custom firmware file.
That’s it! you are done.
Downgrade iOS 5.1 to iOS 4.3.5 and below
The following instructions applies to iPad 2, iPhone 4, 3GS, iPad 1, and iPod touches with SHSH blobs saved on Cydia or by using TinyUmbrella for the iOS 4.x firmware you are downgrading to. iPhone 4S again is not supported.
Step 1: Download the iPSW of the firmware version you would like to revert to, as listed below: iOS 4.3.5, iOS 4.3.4, iOS 4.3.3
Step 2: Connect your device to youe PC or Mac, and get into pwned DFU mode using the latest version of Redsn0w (look for Pwned DFU option in “Extras”).
Step 3: Next, you’ll need to edit your hosts file. It can be found in Windows > System 32 > drivers > etc on Windows or just etc on OS X. Back it up somewhere safe, and then open it with a text editor such as Notepad or TextEdit.
Step 4: Add the following the following string to the bottom of the file, refer to the screenshot for further assistance:
Step 5: Open iTunes and select your device on the left pane, click Restore and locate the iOS 4.x iPSW from the first step.
Step 6: Ignore Error 10xx if it pops up, Use TinyUmbrella or Fix Recovery to get you out of recovery mode.
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