Unless it's a pro forma invoice, meaning it has not yet been booked, you can only delete the very last invoice of every fiscal year.
The underlying reason for this is that the automatic numbering of the invoices would be mixed up otherwise. This in turn is related to the rules of accountancy, which state that invoices should always be chronologically ordered. If you want to delete an invoice, but other invoices have been booked after that one, you will need to delete every invoice up to that date.
Delete the invoice by hovering above the arrow on top and clicking 'Delete'.
The exact same counts for credit notes.
Please note that is not possible to delete invoices in bulk.
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