Bootstrap accordion icon

.styled-faq button.btn.btn-link[aria-expanded=true] i.fa-angle-down {
    display: block;
}

.styled-faq button.btn.btn-link[aria-expanded=false] i.fa-angle-up {
    display: block;
}

                  <div class="card-header" id="headingOne">
                    <h2 class="mb-0">
                      <button class="btn btn-link" type="button" data-toggle="collapse" data-target="#collapseOne" aria-expanded="true" aria-controls="collapseOne">
                        Collapsible Group Item #1
                        <i class="fa fa-angle-down"></i>
                        <i class="fa fa-angle-up"></i>
                      </button>
                    </h2>
                  </div>