Source: http://www.elegantthemes.com/blog/community/divi-nation-short-3-super-simple-divi-header-modifications

Make a child theme with a copy of header.php. Replace this code:

img src="<?php echo esc_attr( $logo ); ?>" alt="<?php echo esc_attr( get_bloginfo( 'name' ) ); ?>" id="logo" data-height-percentage="<?php echo esc_attr( et_get_option( 'logo_height', '54' ) ); ?>" />
Code language: HTML, XML (xml)

with this code:

<span style="font-family: 'Open Sans', Arial, sans-serif !important; color: #505050; font-size: 20px; font-weight: bold;"><?php echo esc_html( get_bloginfo( 'name' ) ); ?></span>
Code language: HTML, XML (xml)