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.
Click on the order you would like to edit.
Click "edit order"
If you want to add an item, click on it and add it to the cart.
Click "go to cart"
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"
Your changes have now been processed!
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.