You can do it on the website not the app. mypass.sixflags.com It will be much easier. You may have to register your pass.
Log into your six flag account and go to your account to update or cancel your membership..
Make sure to try and contact "voice" I always prefer, and have them take you through it, cause some of their policies have been updated and some things maybe an easy fix and others not.
Try the app, call or if close enough just go down there and before you go into the park there is a designated area where they will help you.
Maybe try the Six Flags app?
Thanks! Your answer is awaiting moderation.