Edit in JSFiddle

    $(document).ready(function() {
      // Add minus icon for collapse element which is open by default
      $(".collapse.in").each(function() {
        $(this).siblings(".panel-heading").find(".glyphicon").addClass("glyphicon-minus").removeClass("glyphicon-plus");
      });

      // Toggle plus minus icon on show hide of collapse element
      $(".collapse").on('show.bs.collapse', function() {
        $(this).parent().find(".glyphicon").removeClass("glyphicon-plus").addClass("glyphicon-minus");
      }).on('hide.bs.collapse', function() {
        $(this).parent().find(".glyphicon").removeClass("glyphicon-minus").addClass("glyphicon-plus");
      });
    });

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>

<div class="bs-example">
  <div class="panel-group" id="accordion">
    <div class="panel panel-default">
      <div class="panel-heading">
        <h4 class="panel-title">
          <a data-toggle="collapse" data-parent="#accordion" href="#collapseOne"><span class="glyphicon glyphicon-plus"></span> What is HTML?</a>
        </h4>
      </div>
      <div id="collapseOne" class="panel-collapse collapse">
        <div class="panel-body">
          <p>HTML stands for HyperText Markup Language. HTML is the standard markup language for describing the structure of web pages. <a href="https://www.tutorialrepublic.com/html-tutorial/" target="_blank">Learn more.</a></p>
        </div>
      </div>
    </div>
    <div class="panel panel-default">
      <div class="panel-heading">
        <h4 class="panel-title">
          <a data-toggle="collapse" data-parent="#accordion" href="#collapseTwo"><span class="glyphicon glyphicon-plus"></span> What is Bootstrap?</a>
        </h4>
      </div>
      <div id="collapseTwo" class="panel-collapse collapse ">
        <div class="panel-body">
          <p>Bootstrap is a sleek, intuitive, and powerful front-end framework for faster and easier web development. It is a collection of CSS and HTML conventions. <a href="https://www.tutorialrepublic.com/twitter-bootstrap-tutorial/" target="_blank">Learn more.</a></p>
        </div>
      </div>
    </div>
    <div class="panel panel-default">
      <div class="panel-heading">
        <h4 class="panel-title">
          <a data-toggle="collapse" data-parent="#accordion" href="#collapseThree"><span class="glyphicon glyphicon-plus"></span> What is CSS?</a>
        </h4>
      </div>


      <div id="collapseThree" class="panel-collapse collapse in">
        <div class="panel-body">
          <ul class="nav nav-tabs">
            <li class="active"><a data-toggle="pill" href="#home">Home</a></li>
            <li><a data-toggle="pill" href="#menu1">Menu 1</a></li>
            <li><a data-toggle="pill" href="#menu2">Menu 2</a></li>
            <li><a data-toggle="pill" href="#menu3">Menu 3</a></li>
          </ul>
          <div class="tab-content">
            <div id="home" class="tab-pane fade in active">
              <h3>HOME</h3>
              <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</p>
            </div>
            <div id="menu1" class="tab-pane fade">
              <h3>Menu 1</h3>
              <p>Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
            </div>
            <div id="menu2" class="tab-pane fade">
              <h3>Menu 2</h3>
              <p>Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium, totam rem aperiam.</p>
            </div>
            <div id="menu3" class="tab-pane fade">
              <h3>Menu 3</h3>
              <p>Eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae dicta sunt explicabo.</p>
            </div>
          </div>
        </div>
      </div>

    </div>

  </div>
</div>
</div>