If you need to cancel a Sales Order for any reason, you can do so up until the point when it has been marked "Shipped" or "Picked Up". Once that happens, the order is completed and no further changes can be made.
To cancel an order, pull up the Sales Order page and click the trash can icon in the upper right.
This will cancel the order and you should see the following message at the top of the page:
The Sales Order Status will now be "Cancelled".