DFU means Device Firmware Update. DFU mode allows iOS devices to be restored from any state. Apple TV has DFU mode like the iPhone does. Here is how:
1. Open iTunes on your computer.
2. Connect the Apple TV to your computer using a microUSB cable.
3. Hold “menu” and “down” buttons together on your Apple Remote for six seconds until the device reboots (release when you see the LED light on the Apple TV flashing rapidly).
4. Press “menu” and “play” together right after reboot, until a message appears in iTunes, telling you that it has detected an Apple TV in recovery mode. Your Apple TV will now be in DFU mode and you can restore and update firmware.