United Airlines offers complimentary Premier upgrades to its MileagePlus Premier members based on their elite status and availability of seats. The upgrades are processed automatically before the flight departure, and eligible members will receive an email notification if they have been upgraded. The order of priority for the upgrades is based on the member's Premier status level, fare class, and time of request. Members can also use miles to upgrade their flights, but complimentary Premier upgrades are a perk that comes with being a MileagePlus member.