How does a flight upgrade work?

Moving from a lower cabin to a higher one, paid for or given by the airline to frequent flyers when the cabin has room.

Upgrade, explained

Airlines sell upgrades from economy or premium economy to business class, at booking or closer to departure, and give them to frequent flyers when the cabin has room. Whether a ticket can be upgraded depends on its fare class.

An upgrade is never certain until it is confirmed, and frequent flyer upgrades often need both a qualifying fare and an open upgrade seat. Buying business class outright removes the uncertainty.

On a business class trip

A traveler holding an economy ticket to London bids for an upgrade and waits to the day of departure to know; a business class ticket has the seat from the day it is booked.

Related terms

Fare class (booking class)
A one-letter code inside a cabin that sets a ticket's price and rules. Business class has several, often J, C, D, I and Z; each airline uses its own.
Premium economy
A cabin between economy and business class: a wider seat with more legroom and recline, but no bed.
First class
The cabin above business class on some long-haul flights: fewer seats, more space and more service. Many airlines no longer offer it.
Mixed-cabin itinerary
A trip that flies business class on some flights and economy or premium economy on others, priced as one ticket.