Just updated:
<asp:ScriptManager ID="abc" runat="server"></asp:ScriptManager>
<div class="form-group">
<div class="col-lg-offset-2 col-lg-10">
<table>
<tr>
<td>
<asp:Button ID="btnSearch1" runat="server" Text="Search" class="btn btn-successfrm" OnClick="btnSearch1_Click" />
</td>
<td>
<asp:Button ID="btnClear1" Text="Clear" runat="server" CssClass="btn btn-default" OnClick="btnClear1_Click" />
</td>
<td>
<asp:Button ID="btnExportToExcel1" runat="server" Text="Export to Excel" CssClass="btn btn-warning" OnClick="btnExportToExcel1_Click" />
</td>
</tr>
</table>
<asp:UpdatePanel ID="UpdatePanel4" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:GridView ID="gvManageSo1" runat="server" AutoGenerateColumns="False" DataKeyNames="mstsalesorder_id">
</asp:GridView>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="btnSearch1" />
<asp:AsyncPostBackTrigger ControlID="btnClear1" />
<asp:AsyncPostBackTrigger ControlID="btnExportToExcel1" />
</Triggers>
</asp:UpdatePanel>
</div>
</div>