Change h1 to h3 or strong

Hi I have a question,
I want to change the h1 to a strong or h3 http://i.imgur.com/5e06pbZ.jpg (this h1)
Here is my site: http://bit.ly/2jlL1ms
I don't really know which code I'd use. Maybe a code I can add to my child theme functions.php

best wishes