Hi there StefanieHP,
It's important that all your transactions are correctly recorded so that your book can be well balanced. QuickBooks Desktop is a great tool able to help you keep track of amounts owed to suppliers so that you can pay them their balance. I can point you in the right direction for assistance with modifying the discount.
In order to know the best way to edit these accounts retroactively, I recommend reaching out to the QuickBooks Desktop support team. They'll be able to provide you with the ways this modification can be accomplished in order to get those transactions affecting the right accounts. You can reach them using the methods provided in the QuickBooks Desktop software support policies.
If you have any other questions, feel free to reach out here.