You should check the terms and conditions for your prepaid rental. If you can cancel with no penalty, your best bet would be to just do that and book a new reservation for the new times. Keep in mind that prices may be different now than when you booked.

That said, rental companies understand that flights can be delayed or arrive early, so it's generally not a problem to show up a few hours early/late for your reservation if at an airport. It sounds like you'll be picking up 3 hours early and dropping off 3 hours later than scheduled. This shouldn't be an issue, but keep in mind that it's likely to add an additional day to your rental so there likely will be some adjustment at the rental counter.