You can’t easily, because 8.31.2.31546 is the current and approved stable. What you can do is side-load 8.30.2.31202 and disable auto-updates. That should work, although I have not tried it.
I’ll keep using the latest betas, if only to provide feedback.
EDIT: See here for the last working APK Plex for Android - #417 by ManuelPG
