Plex for My Cloud devices uses internal update procedure to offer new software updates. The following shall describe the way how to do this update. Please excuse some German language in the screenshots, the screen content is always the same in any language. The following shows the update procedure for a DL4100 but it is similar to all other supported My Clouds.
If an update is available, you will see the notification line within the Plex web interface int he header line:
Simply click on "Donwload now" to start the download. This link offers the correct version of Plex that fits to your machine:
Store the file on your local drive "as is".
Open the administrative web interface of your My Cloud, open the App Center of the My Cloud and click on "To install an app manually, click here" in the right bottom corner of "Add an App":
Select the path to the downloaded Plex update BIN file, stored on your local drive.
Confirm the re-installation aka update of the installed Plex Server:
After the Plex update was installed successfully, please confirm the success message:
You will be "kicked back" to the App Center and Plex Media Server. The new software version can be seen in the details:
That's it, Plex update is successfully installed.