UpdatePanel opdaterer HELE siden.

Kode 

<asp:Label ID="Label2" runat="server" Text="Label"></asp:Label>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
            <ContentTemplate>
                <asp:Label runat="server" Text="Label" ID="Label1"></asp:Label>
            </ContentTemplate>
            <Triggers>
                <asp:AsyncPostBackTrigger ControlID="Button1" EventName="Click" />
            </Triggers>
        </asp:UpdatePanel>

Kode 

 protected void Page_Load(object sender, EventArgs e)
    {
        Label2.Text = DateTime.Now.ToString();

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        Label1.Text = DateTime.Now.ToString();
    }