There are many reasons which one might force you to downgrade from iOS 5.1 to its older and lower versions on iPhone, iPad, iPod touch. A few of them include battery woes, pinning for an untethered jailbreak etc.So, let us quickly jump to the steps to downgrade iOS 5.1 to 5.0.1.
Firstly, you must ensure that you’ve saved both APTicket+SHSH blobs for iOS 5.0.1 in order to downgrade iOS 5.1 to 5.0.1. Note that you can downgrade only iPhone 4, 3GS, iPad 1 and iPod Touch. The steps will not work on iPhone 4S and iPad 2. Once you have these, you can continue with the following steps:
Step 1: Start with downloading the latest version of iFaith for windows, official stock 5.0.1 firmware.
Step 2: Click on ‘Build *signed* IPSW w/Blobs’ to build a signage firmware file.
Step 3: The SHSH blobs file saved for your current device downgrade must be browsed. Next, select the stock firmware from iFaith of which you’ve to create custom firmware file for. This leads to a new custom firmware file on your desktop containing SHSH blobs incorporated for the downgrade.
Step 4: Lastly, you just need to enter DFU mode using iFaith. Then use iTunes to restore to custom firmware file.
Voila! Downgrade iOS 5.1 to 5.0.1 successful!
Downgrade iOS 5.1 to 4.3.5
Step 1: Download the iPSW of iOS 4.3.5
Step 2: After connecting your device to your PC of Mac, slip it to pwned DFU mode by using latest version of Redsn0w.
Step 3: Edit the host files from Windows > System 32 > drivers > etc. Open in Notepad.
Step 4: Add 126.96.36.199 gs.apple.com to the bottom of file.
Step 5: Select your device on iTunes. Hold Shift (win), click Restore and locate the elected iOS 4.xiPSW.
Step 6: If Error 10xx pops up, ignore it. Use TinyUmbrella or Fix Recovery to kick it to recovery mode.