Grace Days are automatically implemented in Finance Charges which do not assess on Invoices which are not past due. Due Days automatically sets a Due Date on Invoices.
Use the Invoices Search to show invoices and Add(+) a column for Due Date to view.
Change the Date or Terms to change the Due Date of one invoice, or use the Update tool on the Invoices Search to change, or apply newly-changed Terms to many at once.