You can view your invoices and credit notes by clicking on the main Sales tab.
By clicking on the Filter button, you can search for one or more transactions that meet your criteria.
Once you have located the transaction that you wish to edit, you can click on the Transaction ID in order to view the transaction, and make edits if necessary.
An Apply Credit section will be visible if the contact has any outstanding overpayments or credit notes available to offset against the current invoice. For more information see Allocate Credit Note to Invoice or Allocate Overpayment to Invoice.