Heading size doesn't change

Hi!

I've recently changed one of my websites over to the generate press theme. I'm trying to change the font size of the blog titles which is proving difficult. When I go into Appearance - customize, I'm trying to change it through the typography menu. However, there are two typography menus there that are maybe conflicting with eachother and causing the problem? I'm guessing one is for generatepress and one is just the wordpress font amender. The generate press typography options allow me to amend font size for both mobile or computer, but when I amend these, they just revert back to the size they were before?

Thanks