Hertz says on their website:

https://www.hertz.com/rentacar/produ...ge=prepaid.jsp

Changes/Modifications to "Pay Now" rentals are subject to the terms and conditions below.
….

5. Changes to a reservation must be done at Hertz.com using the "Modify/Cancel" option. Any changes to the reservation may impact the rental charges. If a prepaid reservation is cancelled more than 24 hours before the pickup time, a $100 cancellation fee will be assessed. If the prepaid reservation is cancelled within 24 hours before the pickup time, a $200 fee will be assessed. If the customer does not cancel the reservation prior to the time of pick-up and the rental vehicle is not picked up on the rental date, the entire prepaid amount will be forfeited.

Up to two changes may be made before scheduled pickup. If those modifications result in changes to the prepaid amount, the difference will be charged or credited to your credit card. The booking credit card must be presented at the rental counter for confirmation of reservation. Once rented there are no refunds or credits for unused rental days.