To add new Google Font, lets say "Oswald" - https://fonts.google.com/specimen/Oswald you need to find this line of code in functions.php:
$font_url = add_query_arg('family', urlencode('Playfair Display|Montserrat:400,700|Roboto:300,300i,400,400i,700,700i'), "//fonts.googleapis.com/css");
and replace with this code:
$font_url = add_query_arg('family', urlencode('Oswald|Playfair Display|Montserrat:400,700|Roboto:300,300i,400,400i,700,700i'), "//fonts.googleapis.com/css");
on this way, you will include just Oswald Regular weight. If you want to include Oswald Regular and Bold use code like this:
$font_url = add_query_arg('family', urlencode('Oswald:400,700|Playfair Display|Montserrat:400,700|Roboto:300,300i,400,400i,700,700i'), "//fonts.googleapis.com/css");
Now, your new Google Font is included in theme, but you need to make some CSS changes.
Here is a CSS code which you need to add in Dashboard > Appearance > Customize > Additional CSS to replace all "Playfair" font with "Oswald":
.menu-left-text, .post-num, .blog-item-holder h2.entry-title,
.page h1.entry-title, .blog h1.entry-title, .single h1.entry-title,
.nav-previous a, .nav-next a, .archive-title h1, .search .search-title h1,
.grid-item.quote-item, .portfolio-text-holder .portfolio-text,
.taxonomy-title h1, .carousel-slider .slick-slide .item-text a,
.single-gallery h1.entry-title, .big-text, .footer-first-line,
.our-position-holder, h4.widgettitle
{
font-family: 'Oswald', serif;
}