How to add the Instagram to social buttons?

Try to make it in this way:
1. find file customize_options.php (\wp-content\themes\your_theme\includes\customize) and insert the code after social_google as shown on a screenshot:

$theme->field(
        array(
            'name' => 'social_instagram',
            'label' => 'Your instagram profile',
            'type' => 'text',
            'default' => '#Url'
        ));

2. Goto Appearance=>Editor=> find file content-social.php and above paste code:

<?php if ('' != theme_options('social_instagram')) { ?>
        <li><a data-toggle="tooltip" data-placement="top" title="Instagram" class="zocial instagram"
               href="<?php echo esc_url(theme_options('social_instagram')); ?>"></a></li>
<?php } ?>

 3. Now you should see in Appearance=>Customize settings for Instagram social profile – insert link to your profile.