If you've allocated a credit note to the wrong invoice, you can unallocate it and use it elsewhere.

1. Go to Sales and then Sales Invoices.
2. Search for the relevant invoice and open it.
3. Select the '1 payment' link'.
   ▼ Where is the payment link?

   

4. In the Amount column, select the credit note value.
5. From the allocation, deselect the Invoice and Credit note.
6. Select Save.

The credit note no longer applies to the sales invoice, so you can edit or reverse it. The credit note remains on the customer's account.