If you need to change the due date on the invoice, you should do so on the credit line of the client. That is at Main menu > Database > Clients > Select the client > Credit line
Main menu > Database
Select the Client
Credit Line > Select the term (you may edit or create a new term)
Remarks: Due date depends only on the terms, once a sales order or invoice is created they cannot be modified. The changes will be applied to the new ones.