<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<header><h1>тут любой текст</h1></header>
<script>
var h1 = document.querySelector('header h1');
h1.innerHTML = '<a href="/">' + h1.innerHTML + '</a>';
</script>
</body>
</html>