I have a Synology DS412+ running Plex Media Server and have had for a couple of years, I recently wanted to use Plex on an XBOX One and found that I needed to update the Plex version to the latest and greatest beta version from the Plex website and have documented the process which is straightforward in case it helps someone else. As an aside I have found that streaming from Plex via the Xbox One is superb and probably the best quality client so far.
To download the latest version of Plex you will need to have a Plex Pass.
Log into the Plex website and select downloads, then Plex Pass downloads, lastly select the Nas option
In the next windows select Synology on the right hand side, and then select your processor type. If you are unsure about the processor then login to your Synology dashboard and go to “Control Panel\Info Center” and it will telll you under CPU.
This will then download a .spk file which is the Synology package file for the Plex update.
Next login to the Synology server and go to package management, the Plex package doesn’t have a Synology signature so here we need to change the trust settings to “Any Publisher” to allow us to do the install. I would recommend after the install you change this setting back to what it previously was.
Next in the package manager select manual install and then browse to your .spk file you downloaded in Step 2
You will then see the details of the package you are about to install, if these match to what you were expecting select apply to install.
Once installed restart the Plex service and wait a few moments for the Website to restart, then login to the Plex console and go to settings and then server, if all has gone to plan you should see that the Plex version has updated based on the package you installed.