You can simply change the invoice due date by replacing the initial invoice. In order to do this proceed as follows:
1. First of all open the transaction and click on details of the invoice.
2. Click on Actions > Replace:
3. Change the due date of the invoice.
Please note with the invoice replacement you can also change the prices and invoice details. Once the invoice is stored a new invoice will automatically be created. However, the invoice will not be automatically sent to the customer.