How to change the color of the selected menu?

HTML & CSS
1

Hello All,

In my menu,one of the menu headings is selected and the heading as well as the items within it are displayed using a different color(by using CSS class) when I first land on the page.How do I change the selected menu when I click on some other menu heading?

The following is my HTML:


<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">

<head>

<title>Sample Tab Drop Down Menu Page</title>
<script type="text/javascript">
/*<![CDATA[*/
sfHover = function() {
	var sfEls = document.getElementById("nav").getElementsByTagName("LI");
	for (var i=0; i<sfEls.length; i++) {
		sfEls[i].onmouseover=function() {
			this.className+=" sfhover";
		}
		sfEls[i].onmouseout=function() {
			this.className=this.className.replace(new RegExp(" 

sfhover\\\\b"), "");
		}
	}
}
if (window.attachEvent) window.attachEvent("onload", sfHover);

/*]]>*/
</script>
 <style type="text/css">
#nav, #nav ul {
	padding: 0;
	margin: 0;
	list-style: none;
}

#nav a {
	display: block;
	width: 10em;
}

#nav li {
	float: left;
	width: 10em;
}
#nav li ul {
	position: absolute;
	width: 10em;
           	left: -999em;
}

#nav li:hover ul {
	left: auto;
}
#nav li:hover ul, #nav li.sfhover ul {
	left: auto;
}
#nav .selected a
{
    background-color: red;
}

</style>
</head>


<body>

<ul id="nav">
	<li class="selected"><a href="#">fish1</a>
		<ul>
			<li><a href="#">Remoras</a></li>
			<li><a href="#">Tilefishes</a></li>
			<li><a href="#">Bluefishes</a></li>
			<li><a href="#">Tigerfishes</a></li>
		</ul>
	</li>

	<li><a href="#">fish2</a>
		<ul>
			<li><a href="#">Climbing perches</a></li>
			<li><a href="#">Labyrinthfishes</a></li>
			<li><a href="#">Kissing gouramis</a></li>
			<li><a href="#">Pike-heads</a></li>
			<li><a href="#">Giant gouramis</a></li>
		</ul>
	</li>

</ul>


</body>
</html>

In the above HTML I need to change the selected class to Fish2 when it is clicked.How can I do that? Any help will be greatly appreciated.

Thanks.