asp export excel from gridview by specify column - asp.net

Question

I would like to add a button to export excel from ASP gridview.
Group INV into "data" sheet
Change header from INV to REF & LINE to LN into "details" sheet
Add One column header "FLAG" into "details" sheet
Add row 2
Who can help me to modify below code by VB?
Protected Sub btnconvert_Click(sender As Object, e As EventArgs) Handles btnconvert.Click
Dim strDateTime As String = Format(Now(), "yyyyMMdd-HHmmss")
Response.ClearContent()
Response.AddHeader("content-disposition", "attachment; filename=HRS-SO-" & strDateTime & ".xls")
Response.ContentType = "application/excel"
Dim sw As New System.IO.StringWriter()
Dim htw As New HtmlTextWriter(sw)
Response.Write(sw.ToString())
Response.[End]()
End sub
Gridview1
INV LINE QTY
===============================================
TESTINV001 001 200
TESTINV001 002 200
TESTINV002 001 1000
"data"
REF
INV
===============================================
TESTINV001
TESTINV002
"details"
REF LN QTY FLAG
INV LINE QTY FLAG
===============================================
TESTIN001 001 200
TESTIN001 002 200
TESTIN002 001 1000
<div>
<table class="auto-style4" cellspacing="0" rules="all" border="1" id="GridView1" style="border-collapse:collapse;">
<tbody><tr>
<th scope="col">INV</th><th scope="col">LINE</th><th scope="col">QTY</th>
</tr><tr>
<td>TESTINV001</td><td>001</td><td>200</td>
</tr><tr>
<td>TESTINV001</td><td>002</td><td>200</td>
</tr><tr>
<td>tESTINV002</td><td>001</td><td>1000</td>
</tr>
</tbody></table>
</div>
var tablesToExcel = (function () {
var uri = 'data:application/vnd.ms-excel;base64,'
, tmplWorkbookXML = '<?xml version="1.0"?><?mso-application progid="Excel.Sheet"?><Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet" xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet">'
+ '<DocumentProperties xmlns="urn:schemas-microsoft-com:office:office"><Author>Axel Richter</Author><Created>{created}</Created></DocumentProperties>'
+ '<Styles>'
+ '<Style ss:ID="Currency"><NumberFormat ss:Format="Currency"></NumberFormat></Style>'
+ '<Style ss:ID="Date"><NumberFormat ss:Format="Medium Date"></NumberFormat></Style>'
+ '</Styles>'
+ '{worksheets}</Workbook>'
, tmplWorksheetXML = '<Worksheet ss:Name="{nameWS}"><Table>{rows}</Table></Worksheet>'
, tmplCellXML = '<Cell{attributeStyleID}{attributeFormula}><Data ss:Type="{nameType}">{data}</Data></Cell>'
, base64 = function (s) { return window.btoa(unescape(encodeURIComponent(s))) }
, format = function (s, c) { return s.replace(/{(\w+)}/g, function (m, p) { return c[p]; }) }
return function (tables, wsnames, wbname, appname) {
var ctx = "";
var workbookXML = "";
var worksheetsXML = "";
var rowsXML = "";
for (var i = 0; i < tables.length; i++) {
if (!tables[i].nodeType) tables[i] = document.getElementById(tables[i]);
for (var j = 0; j < tables[i].rows.length; j++) {
rowsXML += '<Row>'
for (var k = 0; k < tables[i].rows[j].cells.length; k++) {
var dataType = tables[i].rows[j].cells[k].getAttribute("data-type");
var dataStyle = tables[i].rows[j].cells[k].getAttribute("data-style");
var dataValue = tables[i].rows[j].cells[k].getAttribute("data-value");
dataValue = (dataValue) ? dataValue : tables[i].rows[j].cells[k].innerHTML;
var dataFormula = tables[i].rows[j].cells[k].getAttribute("data-formula");
dataFormula = (dataFormula) ? dataFormula : (appname == 'Calc' && dataType == 'DateTime') ? dataValue : null;
ctx = {
attributeStyleID: (dataStyle == 'Currency' || dataStyle == 'Date') ? ' ss:StyleID="' + dataStyle + '"' : ''
, nameType: (dataType == 'Number' || dataType == 'DateTime' || dataType == 'Boolean' || dataType == 'Error') ? dataType : 'String'
, data: (dataFormula) ? '' : dataValue
, attributeFormula: (dataFormula) ? ' ss:Formula="' + dataFormula + '"' : ''
};
rowsXML += format(tmplCellXML, ctx);
}
rowsXML += '</Row>'
}
ctx = { rows: rowsXML, nameWS: wsnames[i] || 'Sheet' + i };
worksheetsXML += format(tmplWorksheetXML, ctx);
rowsXML = "";
}
ctx = { created: (new Date()).getTime(), worksheets: worksheetsXML };
workbookXML = format(tmplWorkbookXML, ctx);
var link = document.createElement("A");
link.href = uri + base64(workbookXML);
link.download = wbname || 'Workbook.xls';
link.target = '_blank';
document.body.appendChild(link);
link.click();
document.body.removeChild(link);
}
})();