Pretty easy to do:
- Move your files
- Go to “Settings” in Plex (crossed wrench and screwdriver in the top right)
- On the left, under your server name, click “Libraries”
- For each library you moved, click “Edit”
- Select the new location of the library
- Click save
After this, everything should operate pretty much the same, after Plex re-indexes the files and pretty much just matches them to the existing media it had. You may need to do the Plex Dance if things don’t show up correctly.