Add/edit a product, ametafieldnamed as Sell with COD is visible.
Select the Sell with COD checkbox and save the changes.
Note: You can also manage which all products are available on COD in bulk through CSV, once the app is installed you can export the product data, and search for a column "metafields.sell_with_cod", if the product is available on COD mention 1 else mention 0. After making the changes reimport the product CSV.