Editing a Excel template with POI in Netbeans

Question

I have a Excel template which I want to open, modify and save it like a normal book of Excel when I click in a button in a Jframe.

This is the code:

private void jButton3ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    InputStream inp;
    try {
        inp = new FileInputStream("plantilla.xlt");
        XSSFWorkbook wb = new XSSFWorkbook(inp);
        XSSFSheet sheet = (XSSFSheet) wb.getSheetAt(1);
        XSSFRow row = sheet.getRow(3);
        XSSFCell cell1 = row.getCell(1);
        XSSFCell cell2 = row.getCell(2);
         
        cell1.setCellValue(0);
        cell2.setCellValue(10000);

        //FileOutputStream fileOut = new FileOutputStream("plantilla.xlt");
        //wb.write(fileOut);
        //fileOut.close();
    } catch (IOException ioe) {
        Logger.getLogger(DonnéesFE.class.getName()).log(Level.SEVERE, null,ioe);
    }

}

This is new for me so I have a couple questions:

1. I have to put the whole file path to get the FileInputStream? Is the extension .xlt correct? Right now it doesn't find the file. I have tried with the normal .xlsx extension and it finds it (I don't think it found the good file either) but I get this error on the definition of the workbook that I don't understand:

   Exception in thread "AWT-EventQueue-0" java.lang.NoClassDefFoundError: org/apache/xmlbeans/XmlException.`

2. When I finish editing the Excel template how do I save it as a new Excel book? Everybody uses a new file to save the output but I think that won't work here.