You would not be able to change or modify the number of items once you have already placed an order. You can place another fresh order with the desired modifications. However, you can change your shipping preferences in Your Account at any time after placing your order as long as the order hasn't entered the shipping process yet.
It is very simple;-
To change the quantity for items you have not yet added to the cart -
Once you select the "Add to Cart" button and add the item to your shopping cart, enter the desired quantity in the "Quantity" dropdown option on the right side of the page.
To change the quantity for items already added to the cart -
Go to Cart, select the quantity dropdown box to the right of the title and choose the desired quantity. The quantity number for the item and order amount will be updated automatically.
If the entered quantity is not available with us, you'll see an error message.
You can cancel items or orders by visiting the Your Orders section in Your Account.
To cancel orders that aren't yet out for delivery: