Check you manufacturer's recommendations and unplug the unit before doing any work on the unit. Annual maintenance on a dehumidifier typically includes:

Replace or clean the air filter: Remove the access panel and replace or wash the air filter.

Clean the collection pan: The unit will contain a collection pan for holding the condensate water. Even if you empty the pan regularly, you should still wash it periodically with soap and water or a disinfectant in accordance with your manufacturer's recommendation.

Clean the condenser coils: The condenser coils are behind the exhaust grill. Use a vacuum to remove dust on and between the coils.

Lubricate the fan motor: Follow the manufacturer's directions on how to apply oil to the fan motor.

Check fan and motor hardware: Check for any looseness of the fan or motor nuts and bolts and tighten if necessary.

If the unit has a pipe that goes from the collection pan to a drain, ensure that the pipe is connected properly and secured so that water does not leak out.