Take off the plastic cover under the seat on the side where the fan is located. Once you do that, you will see a mesh grille protecting the fan. This collects lint over time. The lint can cover almost the entirety of the grille, severely restricting air flow.
You can pull off the lint with your fingers or a vacuum cleaner. Do not use a blower however as you'll be forcing all the dust collected by the fan into the battery compartment itself.
Additionally, if you want to be a bit extreme with the fan cleaning, you can actually take out the fan and clean the blades as well. The whole assembly is held down with three 10 mm bolts.