how to change innerhtml back

1<div id="d"> old content </div> 
2 
3<script> 
4    // Element stored in a variable for easy access 
5    var div = document.getElementById("div"); 
6 
7    // Save the original content of the div 
8    var old = div.innerHTML; 
9 
10    // Change the content of the div 
11    div.innerHTML = "new content"; 
12 
13    // Reset the original content of the div 
14    div.innerHTML = old; 
15</script>