The path to the iocage jails has changed in FreeNAS 11.3. This post shows the new path, but the instructions are the exact same as they were on FreeNAS 11.2.

1. Download the newest Plex Update via Plex Dashboard Link
2. Extract the file twice so folders can be accessed
3. Rename folder plexmediaserver
   For Plex Pass Subscribers, rename plexmediaserver-plexpass

4. Stop the current Plex Jail
5. Open WinSCP and login to FreeNAS and browse to: /mnt/FreeNAS/iocage/jails/plexpass/root/usr/local/share
6. Rename existing plexmediaserver folder and add _old at the end
7. Copy the plexmediaserver folder you downloaded into the WinSCP window

8. Right-Click on the copied folder and set permissions recursively to 0775
9. Open the plexmediaserver folder and select the file Plex Media Server
10. Click New – Link

11. Name the link Plex_Media_Server and click OK
12. Click the Link you created and select the Console Button
13. Enter the command: chmod -h 775 Plex_Media_Server

Upgrade Complete!