Sometimes, in order to restore your iPhone firmware using iTunes, you need to put iPhone into Recovery Mode. Lets learn how to enter iPhone in Recovery Mode in order to restore its firmware.
Open iTunes and connect the iPhone via USB.
Press and hold the Home button and the Sleep/Wake button at the same time.
Continue holding the both buttons until your iTunes pops up a message telling you that it has detected an iPhone in recovery mode.
Your iPhone screen will look like this: