You can update the fonts shown on your form by navigating to Theme (droplet icon top-right of the form editor).
From here, select the "Typography" tab and choose the style you want to change from the list provided.
You can set the Primary and Secondary fonts under Theme using the respective dropdown menu listed at the bottom of the window (under the heading "Fonts").
When using an Agency plan, you can add custom fonts to Paperform via our Custom CSS feature, available under Theme → UI Elements → Custom CSS. Check out our article on the fonts we support for more information.