The iOS 4.2 beta firmware was released to the developers yesterday. Sometimes you may have some problems or discomfort with your
iPhone or iPod Touch or iPad iOS 4.2 firmware and you may wish to downgrade your device to iOS 4.1, 4.0.2, 4.0.1, 4 . Here are the instructions to downgrade iOS 4.2 to iOS 4/4.0.1/4.0.2/4.1 firmware.

NOTE : This guide is only for those who have saved SHSH to the Cydia Server.

Do Read : Jailbreak iOS 4.2 on iPhone 3GS

1. Download the firmware file to which you want to downgrade iOS 4.2 to, that is, if you want to downgrade iOS 4.2 to iOS 4.1, then download iOS 4.1 firmware.

2. The second step is to edit the Hosts file.

In Windows

• Open Notepad with administrator privileges. Right click in its icon and choose ” Open as Administrator “.

• Navigate to C:/Windows/System32/drivers/etc in Notepad and open the Hosts file.

• Add ” ” at the end of the file and save the file.

In Mac OS X

• Navigate to Go->Go to Folder and enter ” /etc “.

• Using TextEdit, open up the Hosts file.

• Add ” ” at the and of the document, choose Save As, uncheck ” If no extension is provide, use .txt ” and save the file on the desktop.

• Replace the original Hosts file with the new one.

Do Read : Downgrade iOS 4.1 to iOS 4.x

3. Now connect your device to the computer.

4. Select your device in iTunes.

5. Now enter into recovery mode by pressing the power and home buttons for 10 seconds and then release the power button but keep holding the home button until iTunes recognize your device in recovery mode.

6. Now hold the Shift Key in Windows or the Option Key in Mac OS X and click on the Restore button.

7. Browse to the required iOS 4 or 4.0.1 or 4.0.2 or 4.1 firmware file and select it to downgrade iOS 4.2 to that.

8. Now exit recovery mode ( For this, you could use a software called ” Recboot ”) and reboot your device.

NOTE: If you get a 1004 error, ignore it.

That’s it! Now your device with iOS 4.2 will be downgraded to the required firmware.