How do I delete a credit note that has already been closed?

A closed credit note implies that the amount due to the customer has been paid, either through a direct refund or by applying the credits to an invoice. To delete a credit note, you need to cancel the payment associated with it.
Here’s how you do it:

• Scroll down to the Sales module from the left sidebar and click Credit Notes.
• Select the respective credit note.
• To remove the credits applied to an invoice, navigate to the Invoices Credited tab.
• Now, click the Delete icon to remove the credits applied to the invoice.
• To undo a refund created for a credit note, click the Refund History tab and click the Delete icon.

This will convert the credit note back to Open status. Now you can delete the credit note by clicking the More icon and then, Delete.