To change the showtime just go with your tickets before your schedule show time and select new seats for new showtime. There may be a cost difference such as matinee price vs regular prices.
If I recall correctly, fandango tickets are swapped on fandango. Regular tickets can usually be swapped BEFORE the time on the ticket at the box office, keep in mind prices change based on showtimes and movie type {2d/3d/etc}
IF they let you swap them you would have to speak to the manager on duty or if you ordered via Fandango you can do it on the app
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