Please use the following code snippet to achive the required functionality.
Workbook workbook = new Workbook();
Worksheet worksheet = workbook.Worksheets;
Style style = worksheet.Cells["A1"].GetStyle();
style.Number = 49;
If there is any thing needs further explanation, please feel free to reach us.
If you need to set formatting for the whole column, please use:
Style colStyle = sheet.Cells.Columns[colIndex].Style;