In the examples below it is a WD PR4100 but all manually installable models are similar. Thanks to @sa2000 for providing steps.
1. Download the update binary file either directly from https://plex.tv/downloads or using the link provided by Plex Web App as showing here
2. Then on the myCloud Dashboard, select Apps on the ribbon and then click on the Plex Media Server app showing in the installed apps list
In this example, version 188.8.131.5222 was the installed version, shown by the WD mycloud dashboard as 1613722.
3. Click on the
ON button to toggle the switch and stop the Plex Media Server application
4. Then click on
Install an App Manually and locate the downloaded binary file for the new version of Plex Media Server
5. And click Open
It will now download the the new version into the NAS and apply the install
And will detect that there is already an installed version and will prompt with the following
6. Click ok to confirm
And the update will run
7. Refresh the installed apps screen and you will see new version 184.108.40.20678 in this example as installed and launched
8. Then go to Plex Web page eg http://mycloudpr4100:32400/web/index.html# select Settings / Server and and the new version will be seen as running