Orders are able to be cancelled if there hasn't been any accepted payments on them to date. Cancelling an order will override the cart, scheduled events, and payments meaning it will delete them.
Cancelling open orders is very simple and is done within the top section of the order. The first option you see there is "Job State" which shows a drop down arrow beside the word "Open". Simply click on that drop down arrow and you will see the option to cancel the order.
Note: It will not allow you to cancel the order if you have any payments accepted. You would have to refund them all before proceeding with the cancellation.
Cancelled orders will be able to be found within your Order List and by using the filters. There is an area for Job Filters where you will see "Job State" as an option within. Simply click into there and select "Cancelled" to filter by the cancelled orders within your system.