Preheat oven to 425°F. Line a pie dish with one pie crust.
Combine pie filling, cranberries, and walnuts in a large bowl; mix well. Spoon evenly into crust and place second crust on top.
Fold two crust edges together; seal and crimp. Cut several slits in the top crust to allow steam to escape.
Bake for 35-40 minutes. Transfer to a wire cooling rack; cool completely.
Prepare icing by beating together butter, sugar, and flour with an electric mixer. Add milk, vanilla, and cinnamon; beat well. Additional milk may be added to reach the desired consistency. Spread icing evenly over cooled pie.