Test

First script

HTML

<body>
  <p id="one">One</p>
  <p id="two">Two</p>
  <p id="three">Three</p>
  <p id="four">Four</p>
  <p>
    <button onclick="changeId()">Try it</button>
  </p>  
</body>

CSS

#one {
  color: red;
}

#two {
  color: blue;
}

JavaScript

function changeId() {
  var el = document.getElementById('one');
  el.id = 'two';
}