It's great having you here today, @diana.clark,
You must first delete the payment applied to the invoice to clear the credit, and apply the correction for the discount.
While we're unable to edit and delete payments processed in the Merchant Service Center, I recommend contacting our Support Team.
They are available from Mondays through Fridays at 6 AM until 6 PM PT, and Saturdays at 6 AM to 3 PM PT.
To reach them, please visit this link and proceed to QuickBooks Online Payments & Merchant Service Center section to get their most update contact info: Contact Payments Support.
If you have additional questions or concerns about invoice payments or anything, add another comment below. I'm here to further assist and help you at any time. Have a wonderful day!