DFU Mode (short for "Device Firmware Upgrade") is basically a failsafe in case of issues like failed updates. Despite the name, it is often used to downgrade iDevices.
To enter DFU Mode, connect the iPhone to your computer and turn it off. Hold the Sleep and Home buttons for ten seconds, then let go of the Sleep button ONLY (keep your finger on the Home button until recognized as being in DFU Mode).
If done correctly, your computer will identify the device as an "Apple Mobile Device (DFU Mode)" and the iPhone's screen should be completely off (no backlight, no images).