In WooCommerce, used the plugin WC Cancel Order to implement the functionality Cancel Order for COD as well. But when any user requests cancellation, and even after it has been approved by the admin, the product quantity will not restore.
There are two ways to resolve this
If you want to use plugin then this will work for you Plugin
Or you can use WC Cancel Order
With the help of restore cancelled order, you can add a lot of features to your store like customers or admin will be able to restore those orders that might have been canceled by mistake. Further, you just have to click on the uncancel button while adding a wide range of products to his cart. With the help of this extension, you can also restore multiple purchase requests without inserting any key. After you cancel any order mistakenly, you will return to the original state by simply clicking on uncancel button.