I am trying to change the terms of an invoice after it has been created to allow it to now have deferred payments terms, how can I do this?
This is not possible. An invoice is a legal document, so are the terms of which the payment date was set.
If these have now been renegotiated the best option is to credit the invoice and create a deferred payment invoice. This will give both you and your customer a document detailing the dates on which the payments are expected.