To change the icon open this file templatesja_argotplsblocksheader.php
Find this part of code
<?php if($this->countModules('followus')): ?>
<li class="dropdown parent nav-connect">
<a data-toggle="dropdown" href="#" class=" dropdown-toggle">
<i class="icon-plus"></i><?php echo JText::_('TPL_FOLLOWUS') ?>
</a>
<div class="nav-child dropdown-menu">
<div class="dropdown-menu-inner">
<jdoc:include type="modules" name="<?php $this->_p('followus') ?>" style="T3Xhtml" />
</div>
</div>
</li>
<?php endif; ?>
Change the icon-plus with another icon-
You can find icons codes in this file templatesja_argofontsfont-awesomecssfont-awesome.css