when i have use this code in my GridView so now i can update employee detail...and Object reference not set to an instance of an object solved this error
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<asp:TemplateField HeaderText="Emp ID" >
<ItemTemplate>
<%# Eval("EmpID")%>
</ItemTemplate>
<EditItemTemplate>
<asp:TextBox ID="txtEmpID" runat="Server" Text='<%# Eval("EmpID") %>'></asp:TextBox>
</EditItemTemplate>
</asp:TemplateField>
thanks,
Gaurang R.