As you want to change the size for H1 and H2 there are two possible ways to do this:
1) You can change the h1 and h2 font size for desktop and mobile by using the below CSS code in Theme Settings >> Custom CSS >> Custom CSS for desktop and Custom CSS for tablet:
font-size: 15px !important;
font-size: 10px !important;
The font sizes which i used in the code above is just to set an example for you. You can use any other size by changing the size in the above code.
But remember that this code might not applies in some situations because in the theme somewhere heading tags used with custom selector inside the other tags. So probably in that situation the above CSS might not applied.
2) You can also use the Advanced Typography option from the Theme Settings >> TYPOGRAPHY to change the font size of headings by using their custom selector. But remember that you do not have a separate option for desktop and tablet screens to apply this styling.