UNO (Rev3 or earlier), and Mega boards use either an Atmega16U2 or Atmega8U2 chip as a USB-to-serial converter. By setting a board to DFU mode (Device Firmware Update) you can upload USB-to-Serial firmware to the chip.
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Connect the board to your computer
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Find the RESET and GND pins for the ATmega USB-Serial Processor. They are the innermost two pins of the six located close to the USB port.
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Briefly short the pins (follow these instructions for UNO Rev1)
The board will reset to DFU mode.
Short the HWB line (Rev1 models)
If you’re using a Rev1 model, you also need to assert and hold the HWB line:
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Find the two HWB pads on the back of the board.
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Assert the line by shorting the two pads, e.g. with a piece of wire.
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Short the RESET + GND pins.
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Release the RESET + GND pins.
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Release the HWB line.
The board will reset to DFU mode.