How to Get an Upgrade on a Domestic Flight. Most airlines market their premium cabin on domestic flights as "first class," and these are typically spacious armchair-like seats on narrow-body aircraft (those with one aisle). The best way to get upgraded to first class for free is to earn elite airline status.
How to get a free seat upgrade on United?
You're automatically eligible for a Complimentary Premier Upgrade as long as you are a Premier member (the higher your status, the earlier you become eligible for Complimentary Premier Upgrades and are flying on an eligible ticket on any eligible United-operated flight.
How do people randomly get upgraded to business class?
There are many reasons why one gets upgraded to business class: Some airlines always fill up the higher class cabin (mostly US-based airlines like American or United). When there are any seat available, they upgrade the highest ranked frequent flyer passenger in that flight to the available seat.