You can edit your order on your dashboard by scrolling to the order, and clicking "Edit Selections" on the right side. You can use the + and the - icons to add and remove items and you'll see your order total update on the right. Be sure to save your changes.
Keep in mind, if you're just signing up, the selections in your confirmation email may not match the items you saved on your dashboard. Refer to your dashboard for the most up to date reference.