How to Change Payment Term For Invoices Which Are Posted Into GL

As per user guide, it is not allowed to change the Payment Term after a transaction has been posted.

Also, there is no API that allows update to existing transactions.

Note:  Setting profile option "AR: Update Due Date"  to 'Yes', allows you to change the Due Date of the invoice via the Account Details form

For further information please See section: Maintaining Transactions Field Reference