Device Firmware Upgrade (DFU) mode bypasses the current version of iOS installed on your device and lets you restore your device. It's recommended if you are having problems using the iTunes Restore option. Here's how you do it:
- Turn off your device.
- Connect your device to your computer and wait until iTunes starts.
- Press and hold the Sleep/Wake button and the Home button for 10 seconds.
- At the end of 10 seconds, release the Sleep/Wake button but continue to hold down the Home button for another 10 seconds.
- Release the Home button.
- iTunes will recognize the recovery mode (see image) and ask you to confirm restore process.
- Click OK.
- Click on Restore and Update.