If it's a floor model, remove the acess door that covers the section where water could accumulate if a hose is not used. Scrub it in hot water with a few drops of bleach. Rinse with distilled white vinegar, wipe dry and replace. Pull out the filter and wash per manufacturer's directions for your make and model. Do not use again until completely dry, then replace into the unit.
put vinegar in it (fill), let it sit 1/2 hr. then run thru to brake up mineral deposits(DO this outside so you aren't gagged by the vinegar smell). Rinse well & run water thru 2x to clear it. Buy the capsules that you drop in to help with hard water. https://www.amazon.com/gp/offer-listing/B006O34IW8/ref=dp_olp_new_mbc?ie=UTF8&condition=new buy new replacement filters often to keep running well ; scrub it all with toothbrush(to get in all crevices)hot water and bleach to kill scum,algae,bacterial build up-rinse well.these are to keep tank humidifiers clean between deep cleaning https://www.amazon.com/ProTec-PC-1-Humidifier-Cleaning-Cartridge/dp/B000FKJRDG/ref=sr_1_1?keywords=protec%2Bcleaning%2Bball&qid=1554849628&s=home-garden&sr=1-1&th=1 you can also buy anti-microbial filters great for kids/adults with breathing issues
Perhaps this will help: https://homequicks.com/how-to-clean-dehumidifier
Hi Maty Anne: There should be a filter that you can remove and either replace or clean. If you want to clean it, just soak it in vinegar. But sometimes, it's so old and dirty that replacement is the only solution. What type is it, a floor type or on the furnace?