In the Invoices menu, select All invoices. Choose an invoice or invoices from the visible list. Then choose from Bulk actions the Move to Trash option and Apply. Documents will be moved to Trash. You may go to the Trash to Restore documents or Delete Permanently. These actions can be done through "Bulk actions* as well.
You can also delete all visible documents in Trash by clicking the Empty Crash button.
Only when you remove the invoice permanently you will be able to generate a new invoice for the order.
When you delete an invoice make sure to check your invoices numbering in the Invoices menu -> Settings -> Documents because deleting invoices won't adjust the numbering.