In WHMCS, you can manually change the due date of an invoice. Follow these steps to do so:

1. Log into WHMCS: Access your WHMCS admin panel using your username and password.

2. Navigate to Invoices: Click on the "Billing" tab in the top navigation menu, then select "Invoices."

3. Find the Invoice: Locate the invoice for which you want to change the due date. You can use the search and filter options to narrow down the list of invoices.

4. View the Invoice: Click on the invoice number to open and view the details of the invoice.

5. Change Due Date: In the invoice details view, you will see an option to change the due date. It is usually labeled as "Due Date."

6. Select a New Due Date: Click on the calendar icon or enter a new due date manually.

7. Save Changes: After selecting the new due date, click the "Save Changes" or "Update" button to apply the change.

8. Confirmation: You will typically receive a confirmation message indicating that the due date has been updated successfully.

Keep in mind that changing the due date of an invoice may affect your billing cycle and the client's payment schedule. Make sure to communicate any changes to the client and update your records accordingly. Additionally, consider any automated notifications or reminders that may be affected by the change in due date.