This guide only for users who already saved SHSH Blobs using TinyUmbrella or via Cydia by clicking on "Make my life easier" as shown above. If you don't have SHSH blobs of iOS 4.0.1 or older, then you can't downgrade.
Step 1: Download iOS 4.0.1 for iPhone or iPod Touch.
Step 2: Put your iPhone in DFU Mode by following this steps,
- Connect your iPhone to your computer.
- Turn iPhone off.
- Start iTunes
- Hold Power and Home buttons together for 10 seconds or so.
- Release Power button but keep holding the Home button until your computer recognizes a new USB device.
- iTunes will now recognize your iPad.
- Locate "C:\Windows\System32\drivers\etc\" for Windows and "/etc/" directory for Mac, then locate “hosts” file andOpen it in Notepad (for Windows) or TextEdit (for Mac OS X) and add this at the bottom of the file.
188.8.131.52 gs.apple.comAt last it will be like that, then save it
here, launch it and click on "Exit Recovery Mode" then Device will restart automatically.