Plex Media Server version? Displayed in Synology Package Center → Plex Media Server (click on the name, not on “open”) → Installed Version.
DSM version? Synology Control Panel → Info Center.
See FAQ 15: Claiming & Connecting to a troublesome Synology server.
This applies only when running Plex Media Server on a PC, not on a Synology NAS.
If you want to run Plex Media Server on the NAS, then it is not necessary to install it on your PC. You can completely remove it from the PC if desired. See Plex Documentation → Uninstall Plex Media Server.
