Here is the code for writing data to excel file on desktop-
string path = "C:\\Users\\[User_Name]\\Desktop\\[file_name].xls";
#region Used Excel Objects
MsExcel.ApplicationClass excelApp = new Microsoft.Office.Interop.Excel.ApplicationClass();
MsExcel.Workbook excelWorkbook = excelApp.Workbooks.Open(path, 0, ture, 5, "", "", false, MsExcel.XlPlatform.xlWindows, "", true, false, 0, true, false, false); ;
MsExcel.Sheets excelSheets = excelWorkbook.Worksheets;
MsExcel.Worksheet xlws = (MsExcel.Worksheet)excelSheets.get_Item("sheet1");
MsExcel.Sheets anna = excelSheets;
#endregion
xlws.Cells[1, 1] = Convert.ToString(Value);