DFU is Device Firmware Update. You should put yourinto DFU mode If a restore using Recovery Mode doesn’t work. When placing your into DFU Mode it does not load the OS before attempting the restore.
Step 1: Connect your iPhone to your /PC and press power button for few seconds. You will be asked to slide to power off. Slick it to power off your iPhone.
Step 2: Now Press and hold Home button and the Sleep/Wake button at the same time.
Step 3: After 10 seconds release the Sleep/Wake button but keep holding the home button until iTunes pops up with a message telling you that it has detected an iPhone in recovery mode.
Step 4: Once you are in DFU mode you wouldn’t see anything on you scree. It will be BLACK.