How do I change my codes from office.interop.excel to OpenXML SDK?

Hi all members,

I need your urgent help on this as im running out of time, I just understood that microsoft.office.interop.excel is not reccomended to be used on server-side application as exporting data using Microsoft.Office.Interop.Excel is avoided, since it needs application installation on server, is poorly scalable, and also when I tried after deployment I receive run time errors. I understand that OpenXML is great for server application and DOES NOT need excel to be installed to work.  Thus can I know how should I change my codes from microsoft.office.interop.excel to OpenXML SDK?

My current codes actually allows me to export data of 2 gridviews into individual worksheets(1 gridview data in 1 individual worksheet) which I can set the name of each worksheet itself. My codes should also allow it to save into a specified folder in the folder and another pop up save dialogue to allow users to choose to OPEN/SAVE AS (their own desired location). Thus in each export button clicked there should be 2 excel files generated. 

The below is my current codes:

protected void EXPORT_BUTTON_Click(object sender, EventArgs e)
        {
            Microsoft.Office.Interop.Excel._Application app = new Microsoft.Office.Interop.Excel.Application();

            // creating new WorkBook within Excel application
            Microsoft.Office.Interop.Excel._Workbook workbook = app.Workbooks.Add(Type.Missing);
            String DATA1 = "DATA1";
            String DATA2 = "DATA2";
            ExportToExcel(app, workbook, DATA_1, DATA1);
            workbook.Worksheets["Sheet1"].Delete();
            workbook.Worksheets["Sheet2"].Delete();
            workbook.Worksheets["Sheet3"].Delete();
            ExportToExcel(app, workbook, DATA_2, DATA2);
            string FolderPath = ServerName + DirectoryLocation + DirectoryFolder + ExportsFolder;
            var filename = @"EXCEL_" + datetime.ToString("dd-MM-yyyy_hh-mm-ss") + ".xlsx";
            workbook.SaveAs(FolderPath + filename, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
            workbook.Close();
            app.Quit();
            Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
            Response.AddHeader("content-disposition", "attachment; filename=" + filename + ";");
            Response.TransmitFile(FolderPath + filename);
            Response.Flush();
            Response.End();
        }

        public void ExportToExcel(Microsoft.Office.Interop.Excel._Application app, Microsoft.Office.Interop.Excel._Workbook workbook, GridView gridview, string SheetName)
        {
            // see the excel sheet behind the program
            app.Visible = false;

            Excel.Worksheet worksheet = (Excel.Worksheet)workbook.Worksheets.Add();

            // changing the name of active sheet
            worksheet.Name = SheetName;

            // storing header part in Excel
            for (int i = 1; i < gridview.Columns.Count + 1; i++)
            {
                worksheet.Cells[1, i] = gridview.Columns[i - 1].HeaderText;
            }


            // storing Each row and column value to excel sheet
            for (int i = 0; i < gridview.Rows.Count - 1; i++)
            {
                for (int j = 0; j < gridview.Columns.Count; j++)
                {
                    worksheet.Cells[i + 2, j + 1] = gridview.Rows[i].Cells[j].Text.ToString();
                }
            }

        }

Your kind help is appreciated. thanks.