Some ways to qualify for an upgrade through Enterprise car rental is to become a silver, gold or platinum Enterprise Plus member, notes the Enterprise website. The number of upgrades available each year depend on the membership level that the customer has reached.
As of 2015, Enterprise Plus members who have rented a car six times becomes a silver member and receives one free upgrade per year. Those who have rented 12 times or for 40 days become gold members and receive two free upgrades each year. Platinum members receive four upgrades each year. Members reach this level after 24 rentals or 85 rental days.
Another way a customer may receive a complimentary upgrade from Enterprise is if the model of car that was chosen during the reservation is not available, notes the United Services Automobile Association.