You can add a discount to existing bookings you have in your Bokun system.
Locate the booking in the Sales feed and open it, click on the "Invoices" tab and select "Edit invoice".
You can now add discount to the line items in the booking, edit the tax and even add more line items to the booking if needed. Remember to save your changes.
Your new invoice will now appear under the "Invoices" tab. You can always access older invoices on the right hand side of the screen.