To put the iPhone into DFU mode so you can do an iTunes firmware restore follow these steps:
Step One
Open iTunes and connect the iPhone to your Mac.
Step Two
Press and hold the Home button and the Sleep/Wake button at the same time.
Step Three
After exactly 10 seconds release the Sleep/Wake button. Continue holding the home button until you iTunes pops up a message telling you that it has detected an iPhone in recovery mode.
via How to Put an iPhone Into DFU Mode