How can an invoice be deleted?

If an invoice is final, it cannot be deleted anymore and can only be reversed through a credit invoice.

If the invoice is not final yet, you choose to modify the invoice and indicate that it needs to be reopened.

Then you remove the delivery(ies) on account that are linked to this invoice with the option "Unlink assigned delivery" on the delivery tab.
You must remove them one by one. NedFox advises to write down these numbers!

Once all the deliveries on account have been removed from the invoice, you can then delete the invoice itself. Now you need to consider whether you also want to remove the deliveries on account or if they will be invoiced in a new invoice. To also remove the deliveries on account, go to the Invoicing module and select Account Deliveries. Here you search for the numbers of the deliveries on account that you just noted down, and remove them.