Master Page -CSS class change

Question

Dear Friends,

I have a main menu in master page as below 

 

<div id="menu">

<li class="current_page_item"><a href="../123.aspx">123l</a></li>

<li><a href="../Reports.aspx">Reports</a></li>

<li><a href="#">About Us</a></li>

</div>

It will navigate to Reports.aspx ,if we click on Reports,which is using same mater page 

Now how can I make below for new page   

<li ><a href="../123.aspx">123l</a></li>

<li class="current_page_item"><a href="../Reports.aspx">Reports</a></li>

 

so that my current page will get current page attributes

 

Thanks

Praveen