NPOI how to write to an XLSX excel file

Hi,

I make use of the NPOI libs but it can be answered here I think.

Code:

               using (FileStream file = new FileStream(bestand, FileMode.Open, FileAccess.ReadWrite))
                {
                    wb1 = new XSSFWorkbook(file);
// Lots of code

}
wb1.close(file);

When writing I get the errormessage  on closing: Must support writing: BaseOutputStream

Fileacces = readwrite.

tryed Filemode OpenorCreate.

But no results, the same error messgae

Somebody? its is for an existing XLSX file

Full of ideas but don't know how. Coding for fun.