Override shortcode template in a child theme

Hello,
I am trying to overwrite the latest posts shortcode (I want to change the color classes e put the post thumbnail above the title).
I found a post here on the forums that talks about overwritting the functions but it is from over a year ago and the instructions in it don´t apply anymore.
Could you guys point me to where I need to get the code and where to put it in the child theme?
Thank you