asp export excel from gridview by specify column

I would like to add a button to export excel from ASP gridview.

1. Group INV into "data" sheet
2. Change header from INV to REF & LINE to LN into "details" sheet
3. Add One column header "FLAG" into "details" sheet
4. 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);
    }
})();