You can run the following from within your Plex jail or iocage, whichever you’re using.
PMS_Updater.sh -l /path/to/plex/tar.bz2/ -fv
For complete list of options can be found at GitHub - luluhoc/PMS_Updater: Shell script for updating the Plex Media Server inside the FreeNAS Plex plugin for Latest 11.2-U7 Build(Check Readme if you can't access settings after updating).
Please note, I’m having issues loading plugins with 1.15.0.659 including the system plugin. Not sure if this is an isolated issue or not.