Spray a 9x9" baking pan with non-stick cooking spray.Preheat oven to 350 degrees F.In a large bowl, mix together dry stuffing, apple, celery, onion, and mushrooms.Add butter to boiling water and let it melt, then pour over stuffing mixture in bowl. Mix well until all ingredients are moistened.Spread stuffing mixture in prepared pan, then sprinkle with paprika.Let bake for 35-40 minutes or until the stuffing is thoroughly heated.Keep warm until served.