The My Cloud and MY Cloud Duo come pre-installed with an initial release of that beta. I thought all WD devices have the option to manually install an update. Much faster (hopefully), smaller (mostly) and better PMS builds preview (1.15.0.573-123756e9a)
Here is a thread that describes how to manually update on a different WD device. If the My Cloud has that option, the steps should be similar.
Manually updating Plex Media Server on MyCloud NAS
