Heat oven to 350°F (325°F for dark or nonstick pan). In a 13x9-inch pan (or two: 9-inch round pans), melt butter in oven. Sprinkle brown sugar evenly over butter.
Add enough water to reserved pineapple juice to measure 1 cup. Make cake batter as directed on box, substituting pineapple juice mixture for the water. Pour batter over pineapple and cherries.
Bake 42 to 48 minutes (44 to 53 minutes for dark or nonstick pan) or until a toothpick inserted in center comes out clean. Immediately run a knife around side of pan to loosen cake.