First off, what is the brew temp? Depending on the coffee supplier, and how they roast their coffee, and the bean source, and blend his coffee. Most will need between 198F to 206 F for the max flavor. 202F is norm for a full city roast of arabica and robusto blend.. They will be happy to tell you the maximun extration temperature that their coffee needs.
Next you will need a way to measure it that is fast acting and calibrated. Now you remove the top cover and on the left side about 2/3 rds of the way back is a thermostat. With a screw driver you can tweak the temp into spec. Make sure to allow the boiler to recover between test. A little goes a long way, so be gentle.