<span class='btn btn-more'>
 <i class='fa fa-angle-down'></i>
 <i class='fa fa-angle-up hidden'></i>
</span>

 $('.btn-more').click(function(ev){
 
  $(this).children('.fa-angle-down').toggleClass('hidden');
  $(this).children('.fa-angle-up').toggleClass('hidden');
 })

<li class="sub-item">
 <a href="javascript:;">       
 <span class="arrow"></span>
 </a>
</li>

.arrow:before {
 float: right;
 width: 20px;
 text-align: center;
 display: inline;
 font-size: 16px;
 font-family: FontAwesome;
 height: auto;
 content: "\f104";
 font-weight: 300;
 text-shadow: none;
 position: absolute;
 top: 4px;
 right: 14px;
 color: #990;
}
.arrow.open:before{
content: '\f107';
}

$('.nav-item>a').click(function(){
$(this).children('.arrow').toggleClass('open')
})