Unzip and remove the top half of the cover (a.k.a. the white part), and place it in the washing machine with a mild detergent.
You can use a damp washcloth or sponge with warm water and mild soap to treat a spot on the foam. Dab the area with the water and mild soap, pressing down to make sure you get the area well. Be sure to continuously rinse the towel you’re using to make sure the fluid isn't just being pushed around. Do not soak the mattress. When you're done, prop the mattress up and air dry.