Unplug your diffuser and empty it according to the manufacturer's guide. Fill it halfway with water and 1 teaspoon of vinegar. Turn it on for about 10 minutes. Empty the diffuser again.
Wipe down the water reservoir's surface with a cloth damped with a water and vinegar solution. Use a cotton swab with the same mixture to wipe down delicate and hard-to-reach areas.
Let the entire unit dry before use.