Just as a tip for Arduino newcomers, if you want to put the UNO R3 into DFU mode [1], just bridge these two pins together with a jumper or a cable for a moment:

        ____________________
       | o       ···········
    RESET ---> + · ·
    GND -----> + · ·
      _|____
     |      |
     | USB  |
     |______|
       |
       |

Then, if you want to use the dfu-programmer[2], you need to apply a patch[3] to make it work with the R3.

Being myself a complete newbie, I found it time consuming to reach the state in which I knew how and what to do just to upload a new firmware.

[1]: If you want/are interested in playing with Arduino as an USB device.

[2]: http://dfu-programmer.sourceforge.net/

[3]: https://aur.archlinux.org/packages/dfu-programmer/