Few hours back Apple released iOS 4.1 which is compatible with the iPhone 4, iPhone 3GS, iPhone 3G and iPod Touch iPod Touch 2G and 3G. Most users updated their iPhone to OS intentionally or accidentally and now finding way to downgrade back to previous iPhone firmware. After installing iOS 4.1, you will surely lose your jailbreak on your iPhone and iPod Touch, and being latest firmware you might face incompatibility issues with your favorite iPhone apps forcing you to return to old iOS 4.0.1 firmware. 

If for any reason, you upgraded it to iOS 4.1 and would go back to 4.0.1 or iOS 4.0.2, then follow the steps below (for Windows and Mac users).

VERY IMPORTANT:

1. This process is only for those who have already saved the ECIDs with SHSH to Cydia Server for iOS 4.0.1 or iOS 4.0.2 and we are sure that you had saved it before the official release of iOS 4.1
2. Once you update to a firmware with a new baseband, you are stuck on that baseband regardless of firmware downgrade. iOS 4.1 contains a new baseband – you will lose your carrier unlock as provided by ultrasn0w. (follow this guide to downgrade your baseband from 2.x to 1.59.00)

Steps to Downgrade iOS 4.1 to 4.0.1 iOS Firmware

Step 1: First we make a manual change to the hosts file according to the Windows operating system. Navigate to the path “c:\windows\system32\drivers\etc\” and open the hosts file in a notepad or wordpad. MAC users can navigate to the path “/etc/” and open the hosts file with TextEdit.
Step 2: At the end of file add the string “74.208.10.249 gs.apple.com” useful to redirect requests for certificates for authenticity that iTunes will make to the Saurik server. You may follow the steps below to do so.

• Open Notepad with administrator privileges and then right click on the icon of notepad and click on “Open as administrator”
• Now that Notepad is open Go to folder C:/WINDOWS/system32/drivers/etc and open the file
• Add the following line at the end of the document and save it: 74.208.10.249 gs.apple.com

• Navigate to Go -> Go to Folder and enter the path /etc
• Open the file hosts with TextEdit, add the following string 74.208.10.249 gs.apple.com and save the file to your desktop.
• Now return to the previous folder by following the same method and copy the text within this file by replacing the original file (If prompted for a password enter it and click OK)

Step 3: Set up your device in recovery mode.

• Turn off and unplug your device from your computer
• Now press and hold the Home button while reconnecting the USB cable to your iPhone. After iPhone is connected it should power on the device.
• Continue holding the Home button, you will see the iTunes logo appear and then finally a “Connect to iTunes” will be displayed, now is the time to release the Home button.
• Once the logo appeared in iTunes, you can release the Home button and make a normal recovery

Step 4: Now open iTunes and do a normal reset by holding down the SHIFT key (for Windows users) or Alt key (for Mac users) and clicking restore in iTunes

Step 5: Now select the original iOS 4.0.1 or iOS 4.0.2 firmware and wait for the completion.
Voila! you downgraded from firmware iOS 4.1 GM to firmware 4.0.2 / 4.0.1 and you can follow the normal guides to jailbreak firmware iOS 4.0.1 easily. It is important to note that you cannot jailbreak iOS 4.0.2.