Create link Download file

Hi,
I got a problem to create link Download file, I use LinkButton in UpdateControl. I was make condition such as if the file download is not exist, error message successfully show, but if file download already exist,file download must be downloaded. But what i have done, the file download cannot downloaded. 
this is a example soucecode:
ASP
<asp:UpdatePanel ID="UpdatePanel7" runat="server"> <ContentTemplate> <asp:LinkButton ID="LinkButton1" runat="server">LinkButton</asp:LinkButton> </ContentTemplate> </asp:UpdatePanel> VB
Protected Sub LinkButton1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles LinkButton1.Click Try Dim filename As String filename = "Template SO RT.xls" 'Dim aaa As String = Server.MapPath("~/UploadControl/D ...