How can I save the value of a grid cell in a size(n) grivdiew?

How can I save the value of a grid cell in a size(n) grivdiew?

I have a gridview that is populated from a custom binding to a excel
spreadsheet the sheet would have varying columns and rows and editing is
triggered by on row select. I would like to update the a cell and post the
updated data back to the DataTable for export via excel.
<form id="form1" runat="server">
<div>
<asp:ScriptManager runat="server" />
<asp:UpdatePanel ID="grdUpdatePanel" runat="server"
UpdateMode="Conditional">
<ContentTemplate>
<asp:GridView ID="gridViewTest" runat="server"
OnRowDataBound="gv_RowDataBound"
OnRowEditing="gv_RowEditing"
OnRowUpdated="gv_RowUpdating">
</asp:GridView>
</ContentTemplate>
</asp:UpdatePanel>
</div>
<br/>
<div>
<asp:Button ID="btnExport" runat="server" Text="Export"
onclick="btnExport_Click" />&nbsp;&nbsp;
</div>
</form>
public partial class ExportAndImportExcel : System.Web.UI.Page
{
private DataTable dt;
private DataTable dtab
{
get { return dt; }
set
{
if (dt == null)
{
dt = new DataTable();
}
dt = Session["dt"] as DataTable;
}
}
protected void Page_Load(object sender, EventArgs e)
{
// Disable Export button
this.btnExport.Enabled = false;
loadSpreadsheet(); //Bind data to gridview
}
#region Serialize operation
/// <summary>
/// Serialize excel data stored in datable
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void Serializer() {
JavaScriptSerializer serializer = new JavaScriptSerializer();
String strSerializedText = serializer.Serialize(from a in
dt.AsEnumerable() select a.ItemArray);
}