There are duct cleaning companies who have an assortment of long brushes and large vacuums . Whether you call them, or do it yourself, the first step is to try to remove the actual 'solids' i.e. dust build up, etc. It would be nice to wash down with a odor control solution, but that may not be possible on an extensive duct network. A final step would be to place a Queenaire Ozone generator at the return air inlet, run the system on 'fan' for a few hours. Note that humans and pets cannot be in the building during this time. When completed, a quick airing out dissipates the ozone.