Set value using jquery.

Some time there is a situation where user can create the multiple user control on single page. The user controls have the pop up control from where user can select the employee. Now the pop up control have the label control. Now user need to change the text/label of the header control of the pop up control. at that time the aspx page render the last user control text. Then if am write the JavaScript function for set the value of the label by using getelementbyId then I not able to find the id of the label.

I need to write jquery for set the text of the pop up control header. By finding the class of that label

<script language="javascript" type="text/javascript">
    
    function ChangePopUpHeader(headerString)
    {  
     $('span.popupheader').each( function() {     
         $(this).html(headerString);                 
    });
    }
</script>
 

ScriptManager.RegisterStartupScript(this, this.GetType(), "onclick", "window.parent.parent.parent.parent.window.ChangePopUpHeader('Book Appointment');", true);