Entering Device Firmware Upgrade(DFU) mode
Restoring a device after it enters DFU mode returns it to a known good state with the certainty that only unmodified Apple-signed code is present.
DFU mode can be entered manually:
First connect the device to a computer using a USB cable,
then hold down both the Home and Sleep/Wake buttons.
After 8 seconds, release the Sleep/Wake button while continuing to hold down the Home button.
Note: Nothing will be displayed on the screen when the device is in DFU mode. If the Apple logo appears, the Sleep/Wake button was held down too long.