Place sweet potatoes in large pot, add water to cover sweet potatoes. Bring to boiling. Cover and cook 20 to 25 minutes or until tender, let cool.
Preheat oven to 375 degrees. Pell and mash sweet potatoes. Stir in cranberry sauce and cinnamon. Spoon into lightly greased 2-quart casserole.
Combine flour, sugar, and oats, cut in butter with pastry blender until mixture resembles coarse crumbs. Spoon over sweet potato mixture. Bake 20 minutes. Top with marshmallows and bake 5 minutes or until golden brown.