DFU (Device Firmware Update) mode is a deeper restore mode that can help fix serious software issues on your iPhone. To enter DFU mode, connect your iPhone to a computer with iTunes, then press and hold the power button and the volume down button (or the home button for older models) for 10 seconds. Release the power button but continue holding the other button until iTunes detects your iPhone in recovery mode. Follow the on-screen instructions to restore your device.
