<!DOCTYPE html>

<html>

<head>

<title>Sayfa başlığı</title>

</head>

<body>

<div id="kutu" style="width:300px; height:100px; background:red"> 

<button onclick="renkDegistir()"> renk değiştir </button>

</div>

<script>

function renkDegistir(){

document.getElementById('kutu').style.background="green";

}

</script>

</body>

</html>