Void the old order using the red X button. Keep this tab open.
In a new tab, find the correct event and place a new order. Copy customer info from the previous tab. (Or, for the same event, click the plus icon on the order detail page for an addon order, which retains customer info.) Place the order with cash.
Click “View full order details” and remove the fake cash payment.
Click the money/arrow icon, and copy the previous order number into the field.
Reconcile any price difference. For a balance due, click the money/plus icon and enter payment information.