Upholstery cleaning machine works great....be sure to extract all the water or you will have a water stain. If the mildew persists you can try using some tilex spray, scrub the area and remove any residual liquid with cleaning machine, but ALWAYS test it first in an area that doesn't show. You don't want bleach stains on your header. Also make sure that it's a well ventilated area....those bleach fumes can be awful on your eyes, etc...
When we bought our current boat, the headliner fabric in the forward cabin was all spotted with mildew. It was a miserable job because of the fumes, but well worth the effort.
Lady C