Sometimes iTunes restore doesn’t work in “Recovery mode” so, as a last resort, you have to turn to DFU mode to perform an IPSW restore via iTunes. DFU stands for “Device Firmware Update” and is used to restore the firmware to iPhone/iPod Touch using Apple iTunes. Here is simple step by step process on how to put iPhone into DFU mode:
Step 1: Run iTunes and connect your iPhone to the computer
Step 2: Press and hold iPhone “Home” and “Sleep/Wake” button simultaneously.
Step 3: After exactly 10 seconds, release the “Sleep/Wake” button and keep holding “Home” button.
Step 4: Now iTunes will detect iPhone in recovery mode and a pop up will appear on iTunes:
That’s it. Your iPhone is now in DFU mode and you can follow iTunes restore process to restore the firmware on your device.
Please note that your iPhone screen should remain blank as the given message pops up on iTunes. If your iPhone screen shows the Apple logo instead, it means your iPhone is in recovery mode, not it DFU mode. The reason for that would be that you possibly held the “Sleep/Wake” button for too long. Try releasing it early next time. You may have to try the process again and again to master it.