Sometimes the type of car you booked is not available when you arrive at the pick-up location. Rental companies overbook their cars for the same reason airlines overbook flights: a certain percentage of the people who made a reservation never shows up or cancels their reservation at the last second.
If the type of car you rented is not available, you have the right to get a car that's the same or better. Some rental companies give you a free upgrade, some charge a small amount for an upgrade and some try to rent you a more expensive car anyway.
When you are not satisfied with the solution the rental company offers you, call the customer support department of the broker where you made your reservation. Often they are in a much better position to negotiate with the rental company, because they make many reservations.
After you accepted an alternative the rental company offered you, there is nothing the broker can do about it anymore. So give them a chance to solve problems for you and call their customer support number if any problems occur.