NOTE: Before continuing, verify that you have the SHSH blobs for the firmware you're going to be downgrading to, without them this won't work.

STEP 1: Connect your iPhone into your PC and then put it in DFU mode. To put the device in DFU mode first turn it off, press and hold Home and Power buttons together for about 10 seconds, let go of the Power button while holding the Home button for another 10 seconds, then if the screen goes completely blank, you are in DFU mode.

STEP 2: Navigate to Windows > System32 > drivers > etc on PC.

C:\Windows\System32\drivers\etc

On Mac, navigate directly to etc.

STEP 3: Locate a file named hosts and make a backup copy of it then open it up with a text editor.

STEP 4: Add this code to hosts and save the file:

74.208.105.171 gs.apple.com

STEP 5: Click on iTunes and select your iOS device from the sidebar. While holding the Shift on Windows or Alt on OS X key, select Restore and choose the firmware file which you wish to downgrade to (download all iOS version from here).

iTunes will till you that it will talk to Apple servers but actually it will talk to Cydia servers.

STEP 6: If you receive Error 10xx after doing the previous steps, you may need to use tools like TinyUmbrella or RecBoot to get out of DFU mode.

Now finally youve downgraded your device successfully. Dont forget to revert the changes you made to hosts file in Step 4 above to receive future iOS updates normally from Apples servers.