To edit the social links in the footer, go to Content > Links Lists > "Footer - Social Links"

The social media icons in your site's footer make use of a special group of square, branding icons produced by Font Awesome. To update these icons or see other possibilities, visit https://fontawesome.com/icons and do a quick search. Once you've found the desired icon, copy the provided 'class' information and paste it into the appropriate link list item—in the custom icon field (Linklist is titled, Footer - Social Links)."