Click on the "Shopping Cart" in the upper-right corner of your screen. This will allow you to view all items currently available in your cart as well as the quantities that you have chosen for each item. To change the quantity of an item in your cart, move your cursor to the box that appears under the "Quantity" header for this item. Type in the quantity you want, then click the "Update Cart" link next to the "Price". Once you do this, the quantity and subtotal will automatically update.