Update panel is used when you are working with some of the asp.net controls like dropdown box, buttons and you don't want the whole page to get refreshed after each event then put that section of controls inside the update panel. And script manager is used to support the update panel. You can define script manager directly from the .net tool box-
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
And update panel you can use like this-
<asp:UpdatePanel runat="server" id="UpdatePanel" updatemode="Conditional">
<Triggers>
<asp:AsyncPostBackTrigger controlid="UpdateButton2" eventname="Click" />
</Triggers>
<ContentTemplate>
<asp:Label runat="server" id="DateTimeLabel1" />
<asp:Button runat="server" id="UpdateButton1" onclick="UpdateButton_Click" text="Update" />
</ContentTemplate>
</asp:UpdatePanel>