Access the typography settings
With Reach, fonts can be set for your store's Primary font, Secondary font, and Button font.
To access these, go to Customize > Theme settings > Typography
Select a font
To assign a font to a text category, select Change below the current font. In this selector, scroll or search through the available fonts. Select the desired font, then choose a font style (e.g. Regular, Italic, Bold).
Add custom fonts
To add a font not included in Shopify's font library would require some custom code modifications to the theme files that are beyond the scope of Pixel Union's theme support services.