• Once you have downloaded your firmware from either the downloads page or elsewhere, make sure you have it placed on your Desktop. (You don’t have to it just easier to find and navigate to it).
• When you have, open up iTunes and plug in your device. It should automatically go to your device summary page. If it doesn’t, click you iPhone or iPod under  the heading ‘Devices’ on the far left.
• Next, click and hold one of these two key presses:- Shift key for Windows, Alt key for Mac and then select the ‘Restore’ or ‘Update’ button depending on what you would like to do. (More details and the bottom of this post)
• A new window should appear. Navigate to the folder or area where you placed the firmware; for this guide it’s the Desktop. Now select the firmware and click ‘open’.
• iTunes should now start to upload and extract the firmware to the connected device. This should take no longer than 20-30 minutes maximum.

• Windows
• Mac

Blackra1n: All Device Jailbreak Tutorial (3.1.2)

• Windows