I guess that is unnecessary, as far as I know, adding “nofollow” to your social media link will not affect your website PR. But, if you insist to do it, you need to customize theme code for that kind of changes.
You need to change it in the plugin itself for both Social Share and Instagram feed.
1) For social share, it is located at : wp-content/plugins/jnews-social-share/class-share-bar.php
Go to line:527 and replace with this code: