if you have the server logged into your plex account, you can access @ https://app.plex.tv/desktop > settings > network > secure connection = disable
alternatively you can go to the web admin using whatever internal ip your server is, the same url where you originally setup the plex server and library
for the web client, is in the same place @ settings > web client > general > fall back to insecure connections = always
other clients may or may not have an option for secure connections somewhere in their own options
