FoxGid,
или так
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<script src="//code.jquery.com/jquery-1.11.3.min.js"></script>
</head>
<body>
<p>hello</p>
<p>hello</p>
<script>
// setInterval('alert("Удачи Вам!")', 3000)
$('body > p:first-child').html('New text here');
</script>
</body>
</html>