Hi,
I also have the Odroid HC1 and HC2. They are perfect as an Plex-Server. What I have done to upgrade to the actual version of Plex is to download the version for ARM achitecture ( its something like plexmediaserver_1.16.xxxxxxxxx_armhf.deb)
Then i installed it on the console with :
dpkg -i plexmediaserver_1.16.xxxxxxxxx_armhf.deb --force-all