To change an existing reservation with Allegiant Air, you can visit their website and navigate to the "Manage Travel" section where you can modify your booking by entering your confirmation number and last name. <a href="https://www.allegiantsky.com/b....log/change-existing- to Change the Existing Allegiant Air?</a> .From there, you can make changes to your flight, such as upgrading your seat, changing your flight dates, or canceling the reservation .You may also contact Allegiant Air's customer service for assistance with any changes you wish to make.