How do I delete an invoice once it has been approved and sent to the Accounting System?

All documents, including invoices, can be deleted once they have been reset back to Draft status.

To reset an invoice that has been sent to the Accounting System to a Draft, open the invoice and click on the Options > "Reset Sent to [accounting system name]". 
 

Next, click the Options menu again and select "Reset to Draft".

The invoice will now be in a Draft status and can be deleted by clicking delete symbol in the top right-hand corner.