HOW TO DELETE UNDELETE A INVOICE

That is the question. 
To Delete:
An invoice just click on the invoice in question and you’ll see the Edit and Delete at the bottom of the invoice. 
If the invoice you’re trying to delete sits in VAT period that’s closed. You’ll need to mark that tax period as unpaid, then delete the invoice and then resubmit a new VAT return. This VAT return will have to be done manually.
To UNdelete:
You can’t but you can just recreate that invoice. It doesn’t affect anything.