Code:
<html>
<head>
<script>

function get(){
var x = document.getElementsByClassName('reset');

 for (var i=0; i<x.length; i++) {
    x[i].style.color = 'red'
  }
}
</script>
</head>

<body>
<button onclick="get()">Click</button>

<p class="reset">1</p>
<p class="reset">2</p>
<p>3</p>
<p>4</p>

</body>
</html>