You can delete an invoice as follows:
In the menu, click on 'Invoices' -> 'Received invoices' OR 'Outgoing invoices' (depending on the invoice you want to delete).
You will now see an overview of your (received or outgoing) invoices. Check the box at the far left of the line of the invoice you want to delete.
At the top right of the overview is a dropdown menu with '[Select action]'. The number of invoices you have selected is shown to the left of the dropdown menu. When you click on '[Select action]' you select the option 'Delete'. Next to that you will find the button 'Execute'. When you click this button, the invoice will be deleted.