You can add the login control inside the Update Panel like this.
Ref: Simple Form based authentication example in ASP.Net
<asp:ScriptManager runat="server">
</asp:ScriptManager>
<asp:UpdatePanel runat="server">
<ContentTemplate>
<asp:Login ID="Login1" runat="server" OnAuthenticate="ValidateUser">
</asp:Login>
</ContentTemplate>
</asp:UpdatePanel>