You can easily change the due date on a document. This works for unpaid invoices, receipts and credit notes alike, and the process is the same regardless of the document type.
1. Click the small calendar next to Due date:
2. Then select the new due date from the calendar:
You will then see that the document's due date has changed to the date you selected, and that the document may have moved to a different place in the overview so it appears in the correct chronological order.
You can change a document's due date as many times as you want, but of course you can't change the due date on an invoice that has already been paid.