You can change or cancel the order if the order has not been filled into the market yet (it is still active). 

If you wish to cancel your order, 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 filled 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.