The service override file should be created in: /etc/systemd/system/plexmediaserver.server.d/
And it should be named override.conf. There’s more information (and a specific example for changing users) in this post:
I’d recommend reading through the other steps, but using step E as the means for creating and editing the override.conf file: sudo systemctl edit plexmediaserver (make sure to stop the service first).
