The device is configured by careful selection of R1 and R2. The formula used to select the resistors is:
vTrigger ≈ 2.6 × ( 1 + ( R1 / R2 ) )
For example, using an R1 of 3.9K and an R2 of 3.3K yields a trigger voltage of about 5.67v:
vTrigger ≈ 2.6 × ( 1 + ( 3.9 / 3.3 ) ) ≈ 5.67272727
Another way to calculate the resistors is starting with a known vTrigger and R1 values in order to calculate R2:
( ( R1 * vTrigger ) / 2.6 ) - R1 ≈ R2
For example, I want a vTrigger of 7v and I want an R2 to be a 3.3K resistor:
( ( 3.3 * 7.0 ) / 2.6 ) - 3.3 ≈ 5.58
So my R1 should be 5.58K. Now, it might be hard to find a 5.58K resistor but you at least know the neighborhood you need to be in. It is also important to remember that when you do select your resistor, remember that its tolerance plays a part in determining the actual trigger voltage. So you might not hit exactly on the calculated voltage.
Another way to wire the board for greater flexibility is to use 3.3K resistors for both R1 and R2 but wire R1 inline with a 10k or 20k pot. Combined with an adjustable power supply, you should be able to dial in a desired vTrigger easily.
Lastly, note that I am quite bad with math, so double-check any values you get from this second formula calculation against the first one.