However before jumping into the tutorial, firstly read this requirement first you need to have APTicket + SHSH blobs saved with iFaith,you don't know how to do it? See this video to do so. And iFaith only for Windows, and support the following iDevices, iPhone 3GS, iPhone 4, iPod touch 4g and 3G and the first generation of iPad and Apple Tv 2G. And let's get into the job.
Downgrade iOS 5.1 or iOS 5.0.1 to iOS 5 using iFaith v1.4
1. Download iFaith v1.4 from this link (Mirror).
2. Choose the b 'Build *signed* IPSW w/Blobs', see the following image for the bigger picture:
3. Now get the the Blobs file you saved with iFaith, while browsing for the version of the stock firmware that you select for the iFaith to create the firmware for. Afterward choose ‘Build IPSW’ to make iFaith to start creating a custom firmware using SHSH blobs within for the firmware you wish to downgrade.
4. Continue your step by choosing Proceed and then Start to get your device into DFU Mode using iFaith’s built-in DFU Pwner (iREB).
How to guide to enter DFU mode:
* Press both Power and Home buttons together for about 10 seconds
* Release the Power button but keep pressing the Home till you get the blank screen.
* That means you are in DFU mode.
5. Close iFaith, and then open up iTunes, choose your device type on the left side menu, and on your keyboard please press the left Shift before restoring to your previously saved custom iOS 5.0 IPSW.
Fuihh, now you are in the iOS 5 firmware. It means you managed to downgrade from iOS 5.1 or iOS 5.0.1 to iOS 5 and thanks to iFaith 1.4 to make this possible.