Using a Phillips #2 screwdriver, remove the two screws (circled in red) attaching the fan assembly to the unit.
Carefully disconnect the fan connector from the socket, as depicted in the second picture.
The controller connector panel can be gently pulled out of the main unit and laid down, as depicted. This is not strictly required, but may provide more maneuverability inside the case.
Make sure to not pull out the wires while removing the fan. Doing so may damage the connector wire or the PCB.
Once the fan is removed, use compressed air to clean the dust off of the fan blades.
It is highly recommended that you use a toothpick to prevent the fan blades from moving while cleaning. This will prevent accidental damage to the fans motor.