PREHEAT oven to 400°F. Place Macaroni in 1-1/2-quart casserole dish; stir in boiling water. Cover with lid.
BAKE 25 minutes, stirring after 12 minutes.
STIR in Cheese Sauce and sour cream; sprinkle with shredded cheese. Continue to bake, uncovered, an additional 5 minutes or until shredded cheese is melted.