By default, Folio uses Google fonts. In case, If you want to change font to any other Google fonts, Please have a look in the head part of the webpage and you will find this tag:

<!-- Google Font -->
<link href="https://fonts.googleapis.com/css2?family=Reenie+Beanie&family=Rubik:wght@400;500;700&family=Spartan:wght@400;700;900&display=swap" rel="stylesheet">

To change the font, first go to Google fonts, choose your desired font and use the generated code.

After changing fonts you need to update font families in _variables.scss file

// Font Families
$font-family-base:           'Rubik', sans-serif;
$font-family-heading:        'Spartan', sans-serif;
$font-family-alt:            'Reenie Beanie', cursive;

If you have a need to use custom fonts, Remove above section completely and visit Self Hosted Fonts