Untitled

<!DOCTYPE HTML>
<html>
<head>
    <meta charset="utf-8">
    <title>css</title>

</head>
<body>
<ul id="elem">
    <li>text</li>
    <li>text</li>
    <li>text</li>
    <li>text</li>
    <li>text</li>
</ul>

<script>
    let li=document.querySelector('#li');
    let elem=document.querySelector('#elem');
        li.innerHTML=li.innerHTML+'!';
</script>
</body>
</html>