<ul class="nav nav-tabs">
    <li class="active"><a href="#tab1" data-toggle="tab">Shipping</a></li>
    <li><a href="#tab2" data-toggle="tab">Quantities</a></li>
    <li><a href="#tab3" data-toggle="tab">Summary</a></li>
</ul>
<div class="tab-content">
    <div class="tab-pane active" id="tab1">
        <a class="btn btn-primary btnNext" >Next</a>
    </div>
    <div class="tab-pane" id="tab2">
        <a class="btn btn-primary btnNext" >Next</a>
        <a class="btn btn-primary btnPrevious" >Previous</a>
    </div>
    <div class="tab-pane" id="tab3">
        <a class="btn btn-primary btnPrevious" >Previous</a>
    </div>
</div>

 $('.btnNext').click(function(){
  $('.nav-tabs > .active').next('li').find('a').trigger('click');
});

  $('.btnPrevious').click(function(){
  $('.nav-tabs > .active').prev('li').find('a').trigger('click');
});