var x = $("test");
var y = x.getElementsByTagName("input");
for (var i = 0;i < y.length; i++)
{
    if(y[i].className == "aa")
    {
      y[i].className = "bb";
    }
}

------解决方案--------------------
JScript code

<style type="text/css">
.aa
{
    width:20px;
}
</style>

<input name="a1" class="aa" type="text" /><br />
<input name="b2" class="aa" type="text" /><br />
<input name="c3" class="aa" type="text" /><br />

<input type="button" value="确定" onclick="aa();" />

<script language="javascript" type="text/javascript">
function aa()
{
    var styleSheets = document.styleSheets;
    var rules = styleSheets[0].rules;
    for (var i=0;i<rules.length;i++)
    {
        if (rules[i].selectorText == ".aa")
        {
            rules[i].style.width = "200px";
        }
    }
}
</script>

------解决方案--------------------
用于firefox

<style type="text/css">
.aa
{
   width:20px;
}
</style>

<input name="a1" class="aa" type="text" /><br />
<input name="b2" class="aa" type="text" /><br />
<input name="c3" class="aa" type="text" /><br />

<input type="button" value="确定" onclick="aa();" />

<script language="javascript" type="text/javascript">
function aa()
{
   var styleSheets = document.styleSheets;
   var cssRules = styleSheets[0].cssRules;
   for (var i=0;i<cssRules.length;i++)
   {
       if (cssRules[i].selectorText == ".aa")
       {
           cssRules[i].style.width = "200px";
       }
   }
}
</script>