Cancelling an invoice:
The orders linked to the invoice will be removed from the invoice and will automatically get "ready to invoice status"
Cancelling an invoice means you will loose the numbering of the invoice.
Uncancelling an invoice:
Navigate to the cancelled tab
Open the invoice
And select uncancel invoice option
The status of the invoice will change back to "invoiced"
If you want to use this invoice for another customer/another order, you can adjust the customer on the page and add charge from orders to your uncancelled invoice and regenerate you PDF with correct details: