When you have the install beta file downloaded, put it in the /boot/config/plugins/Plexmediaserver folder.
You will see two files in there similar to:
Plexmediaserver-PhAzE-1.1.4.2757-24ffd60-x64.txz*
Plexmediaserver-PhAzE-1.1.4.2757-24ffd60-x64.txz.md5*
You need to delete the .md5 file, and the old .txz file, and rename your new beta one the same as the one you just deleted. Then force the plugin to install again with:
/etc/rc.d/rc.Plexmediaserver install
Just note that in the settings page GUI it will show an update is available (to the latest stable plex pass version) which you can safely ignore.