Microsoft.Office.Interop.Excel.Application excelApp = new Microsoft.Office.Interop.Excel.Application();
excelApp.Workbooks.Open("c:\\test\\test.xlsx", Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
Microsoft.Office.Interop.Excel.Worksheet sheet = excelApp.ActiveWorkbook.ActiveSheet;
Microsoft.Office.Interop.Excel.Range range = sheet.get_Range("A1", Type.Missing);
range.set_Value(Type.Missing, "test value");
excelApp.ActiveWorkbook.Close(true, Type.Missing, Type.Missing);
excelApp.Quit();