Delete an invoice
Login and go to Sales > Invoices. Filter or otherwise find the one you'd like to delete.
Click on the invoice and you'll see a nice representation of the final invoice that is ready to be sent/was sent to your customer.
Click on the "..." dropdown at the top right of the invoice, and select Delete.
Confirm by pressing OK in the alert that pops up and we'll feed the invoice to a pack of ravenous dogs, tearing it to pieces before digesting it into nothingness.