This is two separate apps.
One is „Plex Web“, the web player bundled with your Plex Media Server. You’re usually running this through your browser via [your PMS IP address]:32400/web or https://app.plex.tv/desktop.
You seem to be using the version installed on your PMS (the hosted version is already newer than that one):
Plex Web - #282 by chrishoage
The Plex Media Player is a separate app that has indeed just been updated to version 2.48. If the built-in update mechanism doesn’t work you can simply download the release from the release notes and drag it into your application folder
P.S.: if your server is still giving you version 3.104 of Plex Web means you haven’t updated your Plex Media Server for almost 6 months 
