Invoices can be voided if they were entered in error, as long as they have not been included in a VAT Report or have not been partially or fully paid/credit noted. A voided invoice still exists as a transaction, but has no impact on your accounting records.
To void an invoice, first open the invoice and then select the Void option from the Other Options menu.
Voided invoices are still visible in the Paid and All Sales tabs, to show that no gaps exist in the invoice number sequence.