You need a file explorer app that’ll allow to install/run other apps (e.g. from the installed Play Store).
Download links to older Plex for Android versions are all over the place in the release announcement thread.
You can put that file on a USB thumb drive or make it available inside your home network (if the file explorer app can also deal with network drives). Just be careful about how far back you go. There’s dependencies and you’ll certainly start losing newer features or bug fixes along the way. Side loading can be helpful if you run in a super specific issue from one version to another – not necessarily a viable long term strategy.
