If you’re going to update to OS5 (which I now regret I did on one of my 2 devices) ssh into the NAS and backup your Plex folders. The update wont delete your app data and folders but when you update the Plex app, things can break due to leaving old files and the downloads on the Plex page arent for OS5. Those will be posted soon hopefully.
If you cant access Plex after the update then turn off Plex, uninstall it in the WD app section, ssh in again and delete both Plex folders. Now install the above linked Plex install, open the config page then turn off Plex again.
Now load your backup folders and files. You will need to rebuild your libraries but will retain your settings.
I wont be upgrading my main NAS to OS5 for a LONG time.