Mix 2 parts of vinegar with 1 part of baking soda in a spray bottle. Allow the fizzing to settle before you place the nozzle on.
Set the Sunbrella up where it will receive direct sunlight for a few hours, but keep it in the down position.
Spray the Sunbrella thoroughly with the vinegar and baking soda. Making sure to get in all the folds and creases. Be especially diligent in spraying the places where you can see the mold.
Open the Sunbrella and spray the underside as well. Let it sit for 10 to 15 minutes.
Brush the Sunbrella with the scrub brush. The underside can be scrubbed in the open position. But the outside will require you to lower the Sunbrella, and brush the material while it is folded up.
Rinse the Sunbrella with a water hose to remove the dirt and mold from the fabric. Then open the Sunbrella and let it dry in full sunlight for several hours. The heat and bright sunlight will kill any remaining mold spores.