How to Downgrade iOS 4.1 GM to iOS 4.0.1 for iPhone 4 / 3GS

Today, Apple announced iOS 4.1 for iPhone and iPod Touch coming next week to public, Also iOS 4.1 GM seed for developers has been released, we showed you our full coverage of iOS 4.1, and we got download links for iOS 4.1 GM so people can try it now instead of waiting a week!

Now, you have upgraded to iOS 4.1 and you want to get back to iOS 4.0.1 or older, here's a quick guide to download iOS 4.1 GM to iOS 4.0.1.



Note:
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.

Steps:

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.

Step 3: This step is about adding Saurik’s Server address to your system hosts file. This trick make iTunes thinks it is talking to Apple, while it is talking to Cydia while the restoring process.

• 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.

74.208.105.171 gs.apple.com

At last it will be like that, then save it

Step 4: Now, You will have to restore your iPhone to iOS 4.0.1 by Clicking on the iPhone icon on the left sidebar of iTunes then hold left “alt” button (“Shift” button on Windows) on the keyboard and then click on “Restore” then release the alt button, Now Select the iOS 4.0.1 ipsw file you have already downloaded in Step 1 and Let iTunes do some automated Steps.

Step 5: You will need to exit recovery mode, so download RecBoot from here, launch it and click on "Exit Recovery Mode" then Device will restart automatically.

Horaya! Now you got back to iOS 4.0.1 and you can jailbreak and unlock your iPhone