Find the IP address for your unit in Plex under Settings, Manage, Live TV & DVR.
Enter the IP address for your device in your browser. The status page will tell you if there is an update available. You might not want to update if your device is in use!
I updated mine also. There was a movie recording that I didn’t care that much about so I went ahead and clicked “update and reboot”. The movie seemed to finish recording undisturbed. I haven’t watched it to see if there are any glitches but doesn’t seem like it actually rebooted after the firmware update.