To delete an invoice go to the Accounting tab -> Invoices. Click on the row of the invoice you would like to delete. The invoice will appear. In the sidebar on the right click the "Delete" button. A pop-up will appear asking you to confirm that you want to delete the invoice. Click "OK" to delete the invoice or "Cancel" if you don't want to delete it.
Note: The default view for the invoice table lists unpaid invoices. If you want to delete a paid invoice you must first select "Paid" invoices in the drop-down menu in the top right corner above the table.