Вы должны использовать метод слияния (xlWorkSheet.Range [xlWorkSheet.Cells [1, 1], xlWorkSheet.Cells [2, 1]]. Merge ();)
Application xlApp;
Workbook xlWorkBook;
Worksheet xlWorkSheet;
object misValue = System.Reflection.Missing.Value;
Range chartRange;
xlApp = new Application();
xlWorkBook = xlApp.Workbooks.Add(misValue);
xlWorkSheet = (Worksheet)xlWorkBook.Worksheets.get_Item(1);
xlWorkSheet.Cells[1, 1] = "Retailer";
xlWorkSheet.Range[xlWorkSheet.Cells[1, 1], xlWorkSheet.Cells[2, 1]].Merge();
xlWorkSheet.Cells[2, 2] = "Brand";
xlWorkSheet.Range[xlWorkSheet.Cells[1, 2], xlWorkSheet.Cells[2, 2]].Merge();
xlWorkSheet.Cells[2, 3] = "2019_WK1";
xlWorkSheet.Cells[2, 4] = "2019_WK2";
xlWorkSheet.Cells[2, 5] = "2019_WK3";
xlWorkSheet.Cells[3, 1] = "LuLu";
xlWorkSheet.Cells[3, 2] = "Perisil";
xlWorkSheet.Cells[3, 3] = "25";
xlWorkSheet.Cells[3, 4] = "26";
xlWorkSheet.Cells[3, 5] = "23";
xlWorkSheet.Cells[4, 1] = "Lulu";
xlWorkSheet.Cells[4, 2] = "Ariel";
xlWorkSheet.Cells[4, 3] = "26";
xlWorkSheet.Cells[4, 4] = "28";
xlWorkSheet.Cells[4, 5] = "29";
xlWorkSheet.Cells[5, 1] = "Danube";
xlWorkSheet.Cells[5, 2] = "Omo";
xlWorkSheet.Cells[5, 3] = "27";
xlWorkSheet.Cells[5, 4] = "28";
xlWorkSheet.Cells[5, 5] = "30";
xlWorkSheet.Cells[6, 1] = "Danube";
xlWorkSheet.Cells[6, 2] = "Tide";
xlWorkSheet.Cells[6, 3] = "24";
xlWorkSheet.Cells[6, 4] = "23";
xlWorkSheet.Cells[6, 5] = "29";
xlWorkSheet.Cells[7, 1] = "Bin Dawood";
xlWorkSheet.Cells[7, 2] = "Persil";
xlWorkSheet.Cells[7, 3] = "26";
xlWorkSheet.Cells[7, 4] = "27";
xlWorkSheet.Cells[7, 5] = "28";
xlWorkSheet.Cells[1, 3] = "Year";
xlWorkSheet.Range[xlWorkSheet.Cells[1, 3], xlWorkSheet.Cells[1, 5]].Merge();
xlWorkBook.SaveAs("test", Microsoft.Office.Interop.Excel.XlFileFormat.xlWorkbookNormal, misValue, misValue, misValue, misValue, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive, misValue, misValue, misValue, misValue, misValue);
xlWorkBook.Close(true, misValue, misValue);
xlApp.Quit();