You can edit an order up until the canteen's cut-off time on the date of the order.
To edit an order:
Go to the "Orders" screen and click on the order you would like to edit.
Click "edit order"
If you want to add more items to the order, click the "Add more items" button.
If you want to remove an item, click on the "x" next to the item. Otherwise, click "edit order" to proceed.
If you are happy with the changes to your order, click "confirm changes"
If your order is less than the original order, you will be credited the difference. If your order is more than the original order, you will be prompted for payment of the difference.