[GUIDE] How to Put Device in DFU Mode and Restore it
What is DFU Mode:
DFU mode is a state that you can put your iPhone into where it can interact with iTunes but does not load the iPhone operating system or boot loader (this is what really differs DFU mode from recovery mode). DFU stands for Device Firmware Update.
What is DFU mode used for?
The primary reason people need to access DFU mode on their iPhone is to change the firmware on the device. This can serve to downgrade to a prior iPhone firmware and operating system, or to use a custom firmware that is required for a jailbreak.
How to enter iPhone DFU mode:
Connect the iPhone to your computer and launch iTunes.Turn the iPhone off (hold down the power button at the top of the iPhone). Hold down the sleep/power button and home button together for exactly 10 seconds, then release the power button. Continue to hold down the Home button until a message appears in iTunes telling you an iPhone in recover mode has been detected by iTunes.
DFU mode is signified by having a completely black screen on the device. If anything else is the case, repeat the steps above until you enter into DFU mode successfully.
Here is a link to video that explains the entire process.