Sprinkle baking soda over the down sofa and let it sit overnight. Vacuum up the baking soda in the morning with the vacuum's upholstery attachment. Baking soda is an all-natural odor absorber and will eliminate much of the stale smell.
Create your own upholstery cleaner by mixing 1 cup of warm water, 1 tsp. of dish soap and one-half tsp. of a water softener. Stir the three ingredients together to form a foam.
Rub the foam into the down-filled sofa fabric until it is no longer visible. Then go over the area again with a damp sponge. Down sofas cannot be soaked with water, but according to Washington State University, the surfaces can be wiped down using this method.
Take the down sofa outside and place it in the sun to dry out. If this is not possible, just set a fan near the sofa to help the upholstery dry a little faster.
Spray the area surrounding the down sofa with white vinegar. The vinegar will eliminate any lingering stale odors that remain in the area.