In the event you need to unallocate a credit note from an invoice, you may either:
1) Go to the credit note you wish to unallocate and click on the 'Less Allocations of' hyperlink.
and then on the Credit Allocations pop-up, click into the 'Delete' hyperlink to remove the allocation.
2) Or you can go to the Invoice that the Credit Note is allocated to and click on the 'Less Credits of' hyperlink.
and then on the Credit Allocations pop-up, click into the 'Delete' hyperlink to remove the allocation.