Change the href of the link element on every web page in JavaScript

In Asp.net web application, I have a user control for showing the header of the applciation on all pages. In the header I have an anchor tag like as:

[code title=””]<div class=”divfield” runat=”server”
id=”divreadinst” style=”padding-left: 20px;
display: none”>
Read the instructions: <a href=”#” id=”lnkURL”
runat=”server” style=”text-decoration: underline;
color: blue; font-style: italic”>Click Here
</div>
[/code]
My requirement is that I need to change the hyper link of lnkURL element according to the requested page.
Can anyone help me to write a JavaScript function to do that?