You need to be on DFU mode to change the firmware of your iPhone, iPad, and iPod Touch. This can serve to downgrade to a prior device firmware and operating system, or to use a custom firmware that is required for a jailbreak or SIM unlock.
How to Enter DFU Mode on iPhone, iPad, and iPod Touch:
Step 1. Connect the device to your computer and launch iTunes.
Step 2. Turn the device off (hold down the power button at the top of the device).
Step 3. Hold down the sleep/power button and home button together for exactly 10 seconds, then release the power button.
Step 4. Continue to hold down the Home button until a message appears in iTunes telling you a device in recover mode has been detected.
When you are in DFU mode your device screen will be completely black.
Even though the screen is black, that doesn’t mean the device can’t be interfaced with either iTunes or a custom firmware client (for jailbreaking, etc).
Remember that if you see the restore logo, iTunes logo, or any message on the screen, you are not in DFU mode but standard Recovery Mode. Again, DFU mode is signified by having a completely black screen on the device. If anything else is the case, repeat the steps above until you enter into DFU mode successfully.