What should be done if you find classified Government Data information NT cleared for public release on the internet?

You don't have to do anything. Whoever posted the data probably let others know where to find it. If you found, it most likely others have found it also and government agencies are already aware of it. Maybe a non-emergency call to a local police station may be appropriate. Forwarding links to the FBI, White House or another government agency may lead to conversations you don't want to have.

On the other hand

How would you know if the information really is secret data. How would you know if the information really is correct or true. Anything can be posted on the internet and you can'r believe everything you read on the internet.