If for some reason you’ve ran into a situation where you want to downgrade your iPhone’s OS but iTunes just won’t comply with your command, I’ve found that there’s an easy way to force the action:

1. Connect your iPhone to the computer and have iTunes opened
2. Hold down both the power and home buttons until the iPhone disappeared from the devices list in iTunes
3. Release the power button but continue to hold down the home button until iTunes detects there’s a device in recovery mode
4. Hold down shift on Windows or option key on the Mac while pressing Restore in the iPhone screen in iTunes to initiate a restore to the desired version of firmware