DFU which stand for Device Firmware Upgrade is a mode that bypasses the current OS installed and allows you to upgrade or downgrade your OS. DFU mode is regularly used to jailbreak your iPhone and it is essential that you are fully familiar with this tricks. DFU is essential for upgrade and downgrade where recovery mode does not work.
DFU Mode is used by many tools as for example:
To get started follow below steps:
Step 1: Make sure your iPhone is plugged into your computer, preferably using the official Apple USB cable for a better connection between the computer and the device. Remember that iTunes must be installed and running on your computer. If you don’t have iTunes installed you can download it from following address.
Step 2: Completely turn off your iPhone and wait for it before proceeding to Step 3.
Now before proceeding to step 3 make sure you are familiar where Home and Power buttons are located on your iPhone; below image should give you a clear picture where they are located.
Step 3: Now press and hold Power and Home buttons together for 10 seconds.
Step 4: At the end of 10 seconds release the Power button while keeping the Home button pressed and keep monitoring your computer screen. Keep the Home button pressed until iTunes does not display a message on your computer
If above message is displayed on your computer screen then your iPhone is in DFU mode.
If you want to restart your iPhone follow below step:
Disconnect and reconnect your iPhone while keeping the Power and Home buttons simultaneously until you restart the iPhone.
Note: DFU mode and recovery mode are different. You will need DFU mode to upgrade/downgrade iPhone or even keeping baseband intact for Unlock. But Recovery mode cant do that. To understand difference between DFU and Recovery Mode and video demonstration, check this Blog post here