The Linux User and Storage configuration article suggests, among other things, editing systemd service files directly to make customizations.
This is bad advice since the official packages from plex.tv overwrite those files at every upgrade (which is expected behavior on systemd-based systems). The correct way to make customizations with the current setup is to create an override file to update any necessary parameters. This is as simple as running systemctl edit plexmediaserver as root and adding any changed parameters.
