It depends. Go back to the website you purchase from. It will be easier that way :)
Talk to a manager
Definitely seek out the instructions according to the website you purchased it from. Otherwise you can take them to the movie theater and see off they'll honor it. ☺
If the website doesn't allow you to cancel them I have bought tickets before and exchange them in the lobby as long as they're not used or past the date that they're for. So if you do not have success online certainly go to the lobby before the movie date on the ticket and exchange it for another movie date, they're very good about it. I hope that helps.
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