Write the code between this
<asp:ScriptManager ID="ScriptManager1" runat="server" />
<asp:UpdatePanel runat="server" id="UpdatePanel" updatemode="Conditional">
<ContentTemplate>
</ContentTemplate>
<Triggers>
<asp:Asyncpostbacktrigger controlid="DropDownList2" eventname="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>