For the header social media links, you have to edit header.php
For eg line 54 of header.php will become
<a rel="nofollow" href="<?php echo get_theme_mod( 'twitter_url', '' ); ?>" target="_blank"><img src="<?php echo get_template_directory_uri(); ?>/img/twitter.png" alt="Follow us on Twitter"/></a>
Social Sharing Buttons are not links but scripts so you cannot nofollow them.