Based on information found here,
If your serial number starts with A (the power button is the last on the right)
Unplug the power cable and, while holding down the second and third buttons (counting from the left), plug in the power cable. Release the buttons either immediately, after five seconds or after the light turns green (apparently whichever works)If your serial number starts with B (the power button is in the middle)
Follow the same procedure, except use the second and fourth buttons