iOS 4.2.1 GM build was released earlier today to the developers. Now if you are on iOS 4.2.1 and want to downgrade your device to iOS 4.1 or 4.0.2 or lower firmware, then here is a step by step guide on how to downgrade iOS 4.2.1 to 4.1 / 4.0.2 or lower firmware on your iPhone, iPod Touch or iPad.

Downgrade iOS 4.2.1

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

1. Download the firmware file to which you want to downgrade your device to, that is, if you want to downgrade iOS 4.2.1 to iOS 4.1, then download iOS 4.1 firmware.

NOTE : If you are having an iPhone 3G or iPod Touch 2G (Non MC model), then skip to step 3.

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 ” 74.208.105.171 gs.apple.com ” 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 ” 74.208.105.171 gs.apple.com “ at the and of the document and save the file to the desktop.
  • Replace the original Hosts file with the new one.

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 4.1 or 4.0.1 or 4.0.2 firmware file and select it to downgrade your device to that.

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

That’s it! You have downgraded iOS 4.2.1 to 4.1 or 4.x.x