Open Theme settings → Typography. Heading font sets every heading and Body font sets everything else, both from Shopify's font library. Heading size, Heading case, Heading letter spacing, Body text size and Button font tune them.
Uploading your own font files is not supported. It is a Theme Store rule, and Shopify's library fonts load faster because Shopify serves them with the right caching. If your brand face is not in the library, pick the closest library face and keep the brand face for your logo image.
More in Typography.