Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<body>

<p id="p1">Hello world!</p>

<script>
function ChangeText()
{
document.getElementById("p1").innerHTML="New text!";
}
</script>

<input type="button" onclick="ChangeText()" value="Change text" />

</body>
</html>

Result:

W3Schools.com - Try it yourself

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