Bootstrap 4巨型汉堡包到汉堡包

Question

<div class="jumbotron col-md-4 lftmargin">
    <div class="anyClass">
        <form action="takeExam" method="post">
            <input type="hidden" name="studentRollNo"                                 value="${session.getAttribute('studentRollNo')}"/>
            <input type="hidden" name="examId" value="${examId}"/>
            <input type="hidden" id="qNo" name="questNo" value=""/>
            <#list questionList as i>
            <button type="submit" id="${i.questionNo}" class="btn btn-secondary change" data-view=${i.questionNo} name="VIEW_INDEX" value="${i_index}">${i_index+1}</button>
            </#list>
        </form>
    </div>
</div>

我有这段代码，当我在移动设备上查看该代码时，我想将巨型机更改为汉堡包图标。