Adjusting the lower limit is a bit trickier. Replace the R2 resistor(s) with a 200K potentiometer (as shown above), connect the desired lower limit voltage to the monitor and adjust the 200K potentiometer until the first red LED just turns on. You can leave this 200K pot in place, but for reliability it is better to measure the resistance across the two potentiometer terminals connected in place of R2 and then replace the potentiometer with fixed resistors totalling the measured value.