When your ambient temperature is lower than your target temperature, you’ll want to use the heating unit to raise the temperature to meet your target.
During the active stage of fermentation (exothermic reaction with the yeast), the temperature can rise by a few degrees.
This is a common occurrence and doesn’t generally affect the beer but if you are worried you could try setting the target a few degrees lower.
Alternatively, for accurate temperature control throughout the whole fermentation process, you can hook the fermenter up to the Grainfather Glycol Chiller.