The connections and process is really similar. I am presuming you have another Arduino board and as such these instructions will be for using that to reprogram the 16u2.
The first step will be to download and upload to your working Uno the
Atmega board programmer sketch by Nick Gammon. That sketch will enable you to reload the firmware of the 16u2, it also supports other boards/processor combo's and is a really good sketch to have on hand.
Having loaded the working board with the sketch the next step will be to connect the board to the 16u2. This can be done as follows: (The image attached can be used to identify the corresponding pins for the 16u2)
Pin 10 : Reset2
Pin 11 : MOSI2
Pin 12 : MISO2
Pin 13 : SCK2
VCC : VCC
GND: GND

Once you have that connected; you will then launch the IDE, open the serial monitor and then connect the "working board" to your computer. You should receive some information from the board identifying the chip that is connected to it (in this case the 16U2) after which you can type "G" to reload the firmware.
More information on how this sketch works can be found on
Nick's website.
Let me know if you need anymore information and I will do my best to help.