You can change or cancel the order if the order has not been finalised yet (they are still active). 

If you wish to cancel your order/purchase click on "More" from your Dashboard then click on My Orders, you can click Cancel all if you wish to cancel all of the orders that have not been finalised yet:


If you wish to cancel an individual order, click on the one that you wish to cancel, a detailed view will open up and here you can click on Cancel order:

You can also change the order if you wish, just click on Change Order: