Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<h1 id="myH1">How to change the style of a header</h1>

<p>Click the button to add a color to the H1 element.</p>

<button onclick="myFunction()">Try it</button>

<script>
function myFunction() {
    document.getElementById("myH1").style.color = "red";
}
</script>

</body>
</html>