What is "firmware" to you?The last sketch you loaded onto the Arduino?The bootloader which enables you to upload sketches without programmer hardware?The Arduino DUE doesn't have a bootloader. The functionality is built in and can't be "downloaded".
Thank you for your replies. Actually we are trying to install NuttX in Arduino Due board by following the instructions given at http://sourceforge.net/p/nuttx/git/ci/master/tree/nuttx/configs/arduino-due/README.txt This link contains procedure to erase flash, writing NuttX to flash, configuring to boot from flash and resetting Arduino Due. Before trying these out on the Due, I want to make sure it is possible to revert Due to it's original state. That's my main intention in asking this question.