If you need to change your order that you already submitted (state: confirmed) or that is already running (state: online), you can stop your order with the "Finish Order" button.
If your account is set to single invoicing, you will immediately receive an invoice that automatically only charges the results you received so far. If your account is set to monthly invoicing, at the end of the month you will only be invoiced for the results you received.
You can easily change and submit your order again by cloning it.