Locate the order you wish to open, click on the ticket amount to bring up Full Review.
A warning will appear, advising you that if the ticket is reopened, it will no longer be settled, and the tendered amount will be voided. Click "Yes". Note, the credit card information will be removed from the ticket so you must obtain it again to charge.
The ticket will load in the menu screen and will be considered open.
The "Reopen History" button will log details of any time a ticket is reopened.
This is controlled by the security settings Cashier Functions > Reopen Closed Ticket.