Most travelers don't know but when you purchase a plane ticket, you usually have 24hrs to change your mind and receive a full refund. That grace period was originally established by the Airlines Reporting Corporation, which handles ticketing services behind the scene for more than 130 airlines. It was adopted long ago to give customers a chance to address any second thoughts or for a travel agency to review and correct any issues.
Most airlines and travel agencies still offer customers 24 hours to cancel and get a full refund. Companies don't go out of their way to inform you of this, which is why I'm here! If you get your ticket receipt via email, read the fine print- you'll most likely have to call for any chances.
Just keep in mind, that if making changes you can only book currently available fares!