Tryit Editor v1.8

Source Code:

<html>
<head>
<script>
function displayResult()
{
document.getElementById("myHeader").innerHTML="Have a nice day!";
}
</script>
</head>

<body>

<h1 id="myHeader">Hello World!</h1>
<button onclick="displayResult()">Change text</button>

</body>
</html>

Result:

W3Schools.com - Try it yourself

Edit the code above and click "Submit Code" to see the result.