DFU (Device Firmware Update) mode is an Apple device mode for connecting with iTunes without loading the operating system or bootloader. DFU mode also works to jailbreak, unlock SIM restrictions, fix non-responsive phones, and upgrade/downgrade firmware. Here's how to get your iPhone or iTouch/iPod into DFU mode.
Step
Step 1. Connect the device to your Mac or PC and select your iPhone from the list of devices in iTunes
Step 2. Turn off iPhone
Step 3. Press and hold the Power and Home buttons simultaneously for 10 seconds (to be precise)
Step 4. Release the Power button but keep the Home button pressed until iTunes displays a message telling you that iPhone is detected in recovery mode
When in DFU mode, the iPhone screen will turn black. If you see the Apple icon or logo, the phone is not in DFU mode but in recovery mode
Step 5. To exit DFU mode, keep the device connected to iTunes and hold down the Power and Home buttons at the same time
Press the Power button to restart the device.