While the various items are within your cart icon (in the upper right side of the screen), you can click on it and a smaller window shall pop up from your right side. Below the items names you will see the option to change the quantity (minimum 1, maximum 10, if the stock allows it) and below the prices you have the remove option. You can then add, remove or change quantities to the items that you want to purchase before proceeding to checkout stage.

After the order is placed, it will not be possible to change the items you purchased.