How can I change the default font for a Jekyll website

Edit the file style.scss located at the root of the jekyll instance, you can change the font from this:

body {
  ...
  font: 18px/1.4 $helvetica;
}

to this:

body {
      font-family: "Times New Roman", Times, serif;
}

or whatever font you want.

If you want to make it better, use variables, add them to _sass/_variables.css

$times: "Times New Roman", sans-serif;

and then change in style.scss:

body {
    font: 18px/1.4 $times;
}

check this Font change Jekyll

The answer you mention telling is to edit your _config.yml file by adding the font names. i prefer add your font in sass file.