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

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

   

4. In the Amount column, select the invoice 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 it or reverse it. The credit note remains on the customer's account.