In order for these instructions to work you must have previously jailbroken on an earlier firmware version and opened Cydia allowing it to save your SHSH on file. Follow steps here for the Mac version
Right click Notepad from within Start:Programs:Accessories.
A popup will appear asking if you would like to give Notepad system access. Click the Yes button.
You are now ready to downgrade your device. Simply put the iPhone into DFU mode and in iTunes Shift+Click the Restore button to select the firmware you would like to downgrade to. If you get error message 1011 or 1013 just ignore it. If you get error message 1015 repeat the entire restore then jailbreak after it fails the second time.