Find the application you actually launched
Identify the running application as Plex HTPC, rather than Plex Media Server, the normal desktop client or a browser tab. Those products have different jobs and update paths. Then inspect the location of the application you opened on the Mac.
Plex’s installation instructions describe dragging the application from its disk image into Applications and launching that installed copy. A copy that opens from another location can still look usable, but the publisher explicitly makes the Applications location part of its automatic-update requirement.
Understand the documented update lifecycle
Plex says the app can detect an update on one launch and apply it after a restart. Keep the currently installed version, detection state and subsequent app restart with the comparison. Immediately reopening a settings panel during the same session is not necessarily the full update cycle described.
Use the publisher’s current distribution route if a manual update is needed. Preserve the existing settings and sign-in information before any broader removal. A successful new download is also different from confirming that the running copy is the one installed in the intended Applications folder.
Report a reproducible location/version result
After following the normal installed-app route, inspect the application’s About information again. Record the version and whether the expected change occurred after the documented restart stage. Avoid keeping several similarly named copies without knowing which one launches from the Dock.
If updating still fails, provide Plex with the Mac version, application version, installation location and the point where the update stops. A programme playback error should be reported separately. This check keeps an installation-location or update-lifecycle issue from becoming an unnecessary media-server rebuild or a change to unrelated library settings.