Connect your phone with iTunes.
If you have lower model from the 8+ Here is the technique for DFU mode
Press both together Power Key + Back Button (Home Button).
When the phone has blank screen then release the Power key but hold Back button (Home Key) for 10 seconds and now you are successfully entered in recovery mode.
Now iTunes Have Following Screen :
Now iTunes Give you the instruction that your phone has in Recovery Mode click on Ok button and move forward to the next Step.
Now we have to select the recovery file or Flash file to Format the iPhone. How to do it -> follow the below steps.
Now you have 2 Buttons on the screen
1 ) Check for Update : This is not for us.
2) Restore iPhone : Here we have to click
Before click on this button i have clear something
If you have the firmware file in your PC so Click on restore button with ” SHIFT” key from the keyboard, Or else do you want to restore online then just click restore button.
Download Firmware : Here
If you have file already then it will take less time.
Now we have following screens while these Process : See in Slide Show.