To put the iPhone5 into DFU mode so you can do an iTunes firmware restore follow these steps
1. Connect the iPhone to your computer and launch iTunes.
2. Hold down the Home key and Sleep - Wake button at the same time.
3. Keep holding both the buttons for exactly 10 secs. Then release the Sleep - Wake button. Keep holding the Home key until the dialog box appears saying " iTunes has detected an iPhone in Recovery Mode"
4. Your iPhone screen will look like this
NOTE : It may take a few attempts to get your iPhone into DFU mode.
Generally, I hold down both buttons then release the Home button just before I think the Apple logo would appear.
If you are still holding both buttons down and you see the Apple logo you are holding them down for too long !
