If you have paid for the order but you want to change one of the products, you have to create a ticket and ask the customer support to change your order. Please indicate the order ID in your e-mail. If you did not pay for your order you can cancel your previous order and make a new one.
If you are not sure about the products you want to buy and you are not ready to place an order you can only add the products in the basket and leave them there as long as you think about what to buy and in what quantity. Because the basket is a flexible place where the client can add and delete the items in an orders as per their needs.