DFU or Device Firmware Upgrade mode allows all devices to be restored from any state. It is a means to getting your iPhone to interface with iTunes without iTunes automatically loading the latest version of iOS onto it (it skips the iBoot bootloader, for jailbreak aficionados).
Differing from recovery mode, where iTunes restores your iPhone to its original factory settings but with the latest iOS and firmware. Thus, DFU mode allows you to change and choose the firmware you want to install onto your iPhone.
For instance, you would use DFU mode if you want to install a previous version of iOS, jailbreak your iPhone or unlock your SIM. If (for whatever reason) standard recovery isn’t working for you to get your device back to working condition, DFU may help you out.
Entering DFU Mode:
- Plug your device into your computer.
- Turn off the device.
- Hold the Power button for 3 seconds.
- Hold the Home button without releasing the Power button for 10 seconds.
- Release the Power Button but keep holding the Home button.
- Keep holding the Home button until you are alerted by iTunes saying that it has detected a device in Recovery Mode.
Exiting DFU Mode:
- Hold Home and Power Button till the Apple Logo appears.