Add sweet potatoes to a large pot of water and bring to boiling. Cover and cook for 20 to 25 minutes or until tender. Preheat oven to 375F (190C). Let cool, and then peel the potatoes. Place peeled potatoes in a large bowl, and mash them. Add the cinnamon and cranberry sauce, combining well. Lightly grease a 2 quart casserole dish and spoon in the sweet potato mixture. In a medium bowl, combine the brown sugar, flour and oats. Cut in the butter until the mixture is crumbly. Spread the mixture over the casserole. Bake for 20 minutes. Top with marshmallows and bake until golden brown, about 5 minutes.