try add this to functions.php
remove_action('wp_enqueue_scripts', 'mesocolumn_theme_load_gwf_styles',20);
this will remove the default open sans font.
however if you want to use different google font, just simply go to customizer->fonts
I am a beginner and have no php knowledge. Where in the functions.php I have to insert that.
I want to stop using Google fonts. Can I set this in the customizer by choosing Headline, Body and Navigation Arial? Will not Google fonts be used anymore?
yes that would do. any window safe font use should eliminate any google font linking.