Starting early Wednesday, United customers won't be able to book flights and access other services as the airline upgrades its reservation system.

United said it would be taking down key online and mobile services for about four and a half hours.

A major airline's bold tech upgrade could leave passengers stranded. Will their plans go smoothly, or will chaos ensue?

United Airlines passengers are facing a big change beginning Tuesday as the airline shifts how early fliers must check-in for trips. If passengers don't abide by the new rules, United says, their ...