You can amend your booking on rentalcars.com using the 'Amend Booking' facility. Simply log in, click on 'My Booking' or 'My Account' and make the changes you want.
There are no administrative fees to amend / change bookings. However, changing your booking may trigger a Rate / Price change, or a charge which may be due from a material change, such as the duration or car type. You can also amend / add your flight number here.