Once a booking is confirmed, you will not be able to change the start time of the booking.
You will only be able to make extensions for the booking (subjected to vehicle availability, of course!). If there is another booking after you, you will not be able to extend your booking and will have to return the car at the specified time. If not, you will incur late booking charges. 😱
For more information on late charges, please refer to this article: