You can change the fonts for different elements on your website at a site-wide and stack-specific level. Please see below on how you can do this:
To change fonts at a site-wide level, click on Change Design in the top left hand corner of your site.
Depending on what you want to change the fonts for, click on the relevant panel and select the font you want:
Changing fonts on a stack-specific level is the same, except you will need to do it from the Stack-specific design panel. Hover over the stack you want to edit and click the Edit Design colour wheel: