DIAGNOSIS: You are asking about maintenance for the primary True HEPA filter. This is a sealed, pleated filter designed for replacement, not cleaning.
FIX: Do not wash or vacuum the main True HEPA filter. Water will destroy the paper media and the structural glue. Vacuuming can tear the delicate pleats. The only user-serviceable cleaning is for the pre-filter, the thin fabric sleeve around the outside of the main HEPA filter. Remove it and vacuum it gently or rinse it with water. Let it dry completely before reinstalling.
IF THAT DIDN'T WORK: If the main filter appears dirty and is causing reduced airflow or odors, it must be replaced. The official replacement part is the Levoit Core 400S Replacement Filter (model number often listed as LV-H134).
HEADS UP: Using a non-genuine or damaged HEPA filter can void the warranty and may allow unfiltered air to bypass the media, reducing purification effectiveness and potentially damaging the fan motor.