How Do I Write To An Existing Excel File?

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C#5.0

I am using the following code to read from excel using NPOI. How do i write to an existing file. Please Help.

     HSSFWorkbook hssfwb;
ISheet sheet;
using (FileStream stream = new FileStream(@filePath, FileMode.Open, FileAccess.Read))
             {
                 hssfwb = new HSSFWorkbook(stream);
                 sheet = hssfwb.GetSheet("Sheet1");
                 //sheet.GetRow(1).GetCell(8).SetCellValue("something");
}
                    var xclDiseaseId = sheet.GetRow(0).GetCell(0).NumericCellValue;