You can toggle the class of the i
element within the clicked anchor like
<i class="fa fa-plus-circle"></i>
then
$('#category-tabs li a').click(function(){
$(this).next('ul').slideToggle('500');
$(this).find('i').toggleClass('fa-plus-circle fa-minus-circle')
});
See working example:
$('#category-tabs li a').click(function() {
$(this).next('ul').slideToggle('500');
$(this).find('i').toggleClass('fa-plus-circle fa-minus-circle')
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
<ul id="category-tabs">
<li>
<a href="javascript:void">
<i class="fa fa-plus-circle"></i> Category 1
</a>
<ul>
<li><a href="javascript:void">item 1</a></li>
<li><a href="javascript:void">item 2</a></li>
<li><a href="javascript:void">item 3</a></li>
</ul>
</li>
</ul>