Workbook workbook = new Workbook();
workbook.LoadFromFile("path");
Worksheet sheet = workbook.Worksheets[0];
sheet.Range["B1"].NumberFormat = "_ * #,##0.00_ ;_ * -#,##0.00_ ;_ * \" - \"??_ ;_ @_ ";
workbook.SaveToFile("accountFormat.xlsx",FileFormat.Version2013);