A Checkout transaction cannot be edited in Full Slate after it is saved, nor can you Checkout an appointment more than once. You can edit the transaction in QuickBooks Online after it syncs there.
To find the transaction in QuickBooks Online, click on that client's profile under the Customers tab. There you'll find each Sales Receipt and Invoice for that client. If you click on the "Checkout" link in the appointment details in Full Slate, you can click to access the sales receipt or invoice.