If you take a look at the LM2596 datasheet at http://www.ti.com/lit/ds/symlink/lm2596.pdf, on page 12 there is a schematic labelled "Adjustable output voltage versions". That board is probably using that design, with R2 replaced by the helical potentiometer. To control the output voltage, I would use a variable voltage applied to pin 4 of the LM2596 through a third resistor. If the control voltage you use is 0 to 5V, then the resistor should be about 3K. You can generate the control voltage from a DAC connected to the Arduino or from a PWM pin and low-pass filter.
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