First, locate all of your home’s vents and clear any furniture, rugs, or other objects out of the area so they don’t get dust on them. After you remove the vent cover, get a vacuum and suck all the dust, dirt, and debris out of the duct. (If the vent is high up or otherwise in an awkward spot, you may want to get someone to help hold the vacuum.) Next, wipe down the inside of the vent with a rag and soapy water. You should also check for mold; if you find any spray with vinegar or mold killing solution and then scrub it away. After you let the duct dry out, replace the cover and put everything back.
Or, if all that seems like a lot of work, you can just hire Brown Service Company to do your duct cleaning for you.