Replace ‘published on date’ with ‘modified on date’ on Posts

Okay, so I have googled this for the past few hours and even though I am getting closer, I am still stuck.

Problem: If I posted an article on December 15th 2016 but I NOW I updated the same article today(July 20th 2017), when you view the article, the updated/modified date is never shown to the user.

NOTE: I did google it and I am able to display the modified date thanks to articles on WPBeginner etc BUT, how do I only post ONE? If the post is updated, it should post JUST the new modified date/time, not both.

I was leaning towards IF and ELSE statements but I can’t figure out how to set it up like this…

* If post was modified, post modified date, else post date it was published. *

If someone can please help me out here, I would appreciate it a lot, I am lost, thanks!

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