IFRAME not updating

 i am using an iframe in the page  like this
<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeFile="Default.aspx.cs" Inherits="_Default" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
<p>
<asp:Button ID="btnPre" runat="server" Text="Previous" onclick="btnPre_Click"
        Width="71px" /><asp:Button ID="btnNext" runat="server" Text="Next"
        onclick="btnNext_Click" />
</p>

<iframe id="centerbox" runat="server" src="About.aspx" marginwidth="0" marginheight="0" frameborder="no" scrolling="yes">

</iframe>

</asp:Content>
and it shows the ...