You don’t need to add UpdatePanel to Table Just add Update panel to DropdownList control.
Refer the below sample code for your reference and implement it as per your code logic.
HTML
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<cc1:ToolkitScriptManager runat="server">
</cc1:ToolkitScriptManager>
<div>
<asp:Panel ID="pnlpopup" runat="server" Width="617px" Height="600px" ScrollBars="Auto">
<table style="background-color: white; text-align: left" width="600px" class="grid_uplod">
<tr>
<th valign="middle" colspan="2" align="center" bgcolor="#FFFFFF" style="font-weight: bold;
font-size: larger">
<asp:ImageButton ID="imgOK" ImageUrl="~/Images/Available.png" Height="20" Width="20"
runat="server" ImageAlign="Right" />
</th>
</tr>
<tr>
<td colspan="2">
<hr />
</td>
</tr>
<tr>
<td style="text-align: left;">
Action :
</td>
<td style="text-align: left;">
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Always">
<ContentTemplate>
<asp:DropDownList ID="DDLRemedialAction" runat="server" class="form-control" AutoPostBack="true"
OnSelectedIndexChanged="DDLRemedialAction_SelectedIndexChanged">
<asp:ListItem Text="Please select" Value=""></asp:ListItem>
<asp:ListItem Text="1" Value="1"></asp:ListItem>
<asp:ListItem Text="2" Value="2"></asp:ListItem>
</asp:DropDownList>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="DDLRemedialAction" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>
</td>
</tr>
<tr>
<td>
Attachment :
</td>
<td>
<cc1:AjaxFileUpload ID="AjaxFileUpload11" runat="server" MaximumNumberOfFiles="10"
Width="100%" OnUploadComplete="OnUploadComplete" />
</td>
</tr>
</table>
</asp:Panel>
</div>
</form>
</body>
</html>
C#
protected void OnUploadComplete(object sender, AjaxControlToolkit.AjaxFileUploadEventArgs e)
{
// Your Code
}
protected void DDLRemedialAction_SelectedIndexChanged(object sender, EventArgs e)
{
// Your Code
}
VB.Net
Protected Sub OnUploadComplete(ByVal sender As Object, ByVal e As AjaxControlToolkit.AjaxFileUploadEventArgs)
'Your Code
End Sub
Protected Sub DDLRemedialAction_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
'Your Code
End Sub
Screenshot
