To change the deadline for one invoice:

1. Open the invoice in edit mode
2. In the Invoice Details card, update the "Due date" field
3. Save the invoice

If you want future invoices to start with a different deadline, update the customer's payment terms or the default at Settings → "Facturen en offertes" under "Hoeveel dagen krijgt een klant om te betalen?" (how many days does a customer get to pay?).

Tip: Automatic payment reminders follow the due date, so a correct deadline also means reminders go out at the right moment.