At this time, there isn't an option to edit payment receipts to show credits applied. As a workaround, you can consider entering the amount of credit in the Memo field to show it when printing the receipts.
Here's how:
Go to the Sales menu, then find the payment transaction.
In the Receive Payment window, enter the credit amount in the Memo field.
Click Print.
Once done, the memo will show on your payment receipts stating the credit amount.
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