Went and looked this up for you.
This is from STOREY'S GUIDE TO RAISING CHICKENS by Gail Damerow:
Vacuum up loose down, sponge out hatching debris, and scrub the incubator with detergent and hot water. When the incubator looks thoroughly clean, apply a disinfectant such as Germex or chlorine bleach (1/4 Clorox per gallon of hot water, or 30ml/l). If possible, dry the incubator in the sun.
(Because hatching itself is a major source of contamination, clean your incubator after each hatch. This will improve hatching success and also helps break the natural disease cycle in a flock.)
Hope that helps!