I rarely write poor reviews, however if you book though Hotwire, and prepay for a car, you will not be able to modify your reservation.
Hotwire's website under FAQ, states that you may contact the rental car agency, and request modification. However, not only did Dollar refuse to modify my reservation because it was prepaid, a lady from Dollar said if I were to drop the vehicle off at a different location, Dollar would report it missing/stolen. That's when I terminated the conversation. I will never use or recommend Hotwire or Dollar car rental again.