Preheat oven to 350°. Melt butter in 9 x 13 pan in oven. Sprinkle brown sugar evenly over butter. Arrange pineapple and cherries over top and press gently. Add enough water to pineapple juice to measure 1 ¼ cups. Make cake batter as directed on package except use pineapple juice mixture in place of water. Pour batter over pineapple and cherries. Bake 40-45 minutes or until toothpick inserted in center comes out clean. Immediately turn pan upside down onto heatproof serving plate. Leave pan over cake for 1 minute. Serve warm or cool. Store loosely covered.