How do I cancel an order?

There are two ways to cancel the order.
You mark the order as canceled in the drop-down list Admin > Site Name > Orders > Status column, then you would need to click the update status button.  
Or you mark it as canceled after opening the item for editing then you would just need to click save afterward.
Admin > Site Name > Edit Order (pencil) > Status tab
That is all you need to do on the Pressero side for it.  In order to receive the refund of payment then you would need to contact your merchant processor account, we do not handle refunds on the Pressero side.
For more details on orders, please review this article: Ch. 004. Orders