How to split three single columns into one big column?

See more:

C#

MS-Excel

I've got this type of data in my Datatable. Imagine that ("2019_WK1", "2019_WK2", "2019_WK3") is the three column names and so i need to take split 2019(year) in one big merged cell, (Upper "WK1", "WK2", "WK3") column cells in C#.

using Excel = Microsoft.Office.Interop.Excel;

Below are my datatable code:

Excel.Application xlApp;
Excel.Workbook xlWorkBook;
Excel.Worksheet xlWorkSheet;
object misValue = System.Reflection.Missing.Value;
Excel.Range chartRange;

xlApp = new Excel.Application();
xlWorkBook = xlApp.Workbooks.Add(misValue);
xlWorkSheet = (Excel.Worksheet)xlWorkBook.Worksheets.get_Item(1);

//add data
xlWorkSheet.Cells[1, 1] = "Retailer";
xlWorkSheet.Cells[1, 2] = "Brand";
xlWorkSheet.Cells[1, 3] = "2019_WK1";
xlWorkSheet.Cells[1, 4] = "2019_WK2";
xlWorkSheet.Cells[1, 5] = "2019_WK3";


xlWorkSheet.Cells[2, 1] = "Lulu";
xlWorkSheet.Cells[2, 2] = "Perisil";
xlWorkSheet.Cells[2, 3] = "25";
xlWorkSheet.Cells[2, 4] = "26";
xlWorkSheet.Cells[2, 5] = "23";

xlWorkSheet.Cells[3, 1] = "Lulu";
xlWorkSheet.Cells[3, 2] = "Ariel";
xlWorkSheet.Cells[3, 3] = "26";
xlWorkSheet.Cells[3, 4] = "28";
xlWorkSheet.Cells[3, 5] = "29";

xlWorkSheet.Cells[4, 1] = "Danube";
xlWorkSheet.Cells[4, 2] = "Omo";
xlWorkSheet.Cells[4, 3] = "27";
xlWorkSheet.Cells[4, 4] = "28";
xlWorkSheet.Cells[4, 5] = "30";

xlWorkSheet.Cells[5, 1] = "Danube";
xlWorkSheet.Cells[5, 2] = "Tide";
xlWorkSheet.Cells[5, 3] = "24";
xlWorkSheet.Cells[5, 4] = "23";
xlWorkSheet.Cells[5, 5] = "29";

xlWorkSheet.Cells[6, 1] = "Bin Dawood";
xlWorkSheet.Cells[6, 2] = "Perisil";
xlWorkSheet.Cells[6, 3] = "26";
xlWorkSheet.Cells[6, 4] = "27";
xlWorkSheet.Cells[6, 5] = "28";

xlWorkBook.SaveAs("d:\\csharp.net-informations.xls", Excel.XlFileFormat.xlWorkbookNormal, misValue, misValue, misValue, misValue, Excel.XlSaveAsAccessMode.xlExclusive, misValue, misValue, misValue, misValue, misValue);
xlWorkBook.Close(true, misValue, misValue);
xlApp.Quit();

releaseObject(xlApp);
releaseObject(xlWorkBook);
releaseObject(xlWorkSheet);

MessageBox.Show("File created !");
}

What I have tried:

I need like this type of columns

____________________________
| 2019 |
|___________________________|
| WK1 l WK2 l WK3 |
|________l________l_________|
| 25 | 26 | 23 |
| 27 | 29 | 24 |
___24________25_______22_____