This evening opening Plex I found the following message.
I went to check on https://hub.docker.com/r/plexinc/pms-docker/ and noticed there was an update from 16 hours ago.
I modified my yaml file by inserting the tag :latest (I know, it wasn't necessary ...) and I restarted the stack.
Now, apart from the fact that I have lost all configuration, the version has not updated.
I also read the post How to update Plex Media Server with Portainer but I believe I have done the correct steps.
How should I upgrade it?