How do I change the fonts?

Fonts are set in one place for the whole storefront. Pick a body face and a heading face, and every section follows them.

Open Theme settings → Typography.
Set Body font for running text and Heading font for titles. Both use Shopify's font picker, so you choose from the built-in library.
Adjust Body text size and Heading size to scale the type up or down, and Mobile heading size to tune headings on small screens.

There's no per-section font override. Changing these two fonts restyles the entire store.