How do I correct cleared Payment Transactions

Our Auditor found some cleared Payment Transactions that are in error.  We did not take advantage of discount terms on some invoices, but the Net Amount in the payment transaction indicates that we did take the discount.

I tried to cancel the payment transaction by re-uploading the remittance .csv files...indicating a CanceledDate and ExternalProcessedState = 1 (indicates cancelled) in remittance.csv.  But when I look at the transaction in the GUI, it appears unchanged.

What else do I need to do to cancel the existing payment transaction before uploaded the corrected one?

TIA...Chuck