Can not put Arduino UNO R2's atmega 16U2 into DFU mode

spcomputing:
After shorting/resetting go to the Device Manager and see if there is an "Unknown Device". If it is an unknown, try to install the driver from the FLIP\usb folder (atmel_dfu_usb.inf).

If not, we will need a programmer to re-install the firmware manually with AVRDUDE.

It is NOT an unknown device, so how do I re-install the firmware manually with AVRDUDE? Can you give me a reference?
Thank you!