The easiest way to do this on macOS is to move the current Plex data folder to the desired location and then create a symbolic link to it in the default location. Stop Plex Media server, then run:
mv "~/Library/Application Support/Plex Media Server" "/new/location/Plex Media Server"
ln -s "/new/location/Plex Media Server" "~/Library/Application Support/Plex Media Server"
Replace “/new/location” with the desired new home for Plex data. Then start Plex Media Server.
I would recommend that the new location not be a network drive. I’d recommend against an external USB drive as well, if it can be avoided. Removable devices aren’t the best choice for your Plex data; it’s easy for them to become disconnected or ejected while Plex is running, which could lead to database corruption.
