You would turn it clockwise a very small amount. It should be marked with temperature settings. But there is usually a reason that the immersion is not getting as hot as it used to. If its an immersion in the bottom of the tank, its possible that the tank has got hard water scale build up & the heater element is sitting in this scale & not heating the water directly. The other possibility is the thermostat needs changing. [b][size=150][color=#FF0000]Remember to turn off[/color] [/size][/b]before adjusting the temperature.