Hello,

yes you can delete or modify invoice if the invoice is in the Draft or canceled stage. once you validate that invoice it goes into a open stage and after giving payment it goes into a paid stage in this two stage you can not delete invoice.

if your invoice  is in the draft of cancel stage  delete it and make your new invoice according to your need this is only way to modify you invoice.

Yes, u cannot delete an invoice which is already paid. Instead of that u can go for refund invoice and click on edit from that option click on action button->drop down it and click on delete or else u can modify the draft refund invoice and again click on save and make the payment.

Hope this helps.