Replacing a text in Apache POI XWPF

The method you need is XWPFRun.setText(String). Simply work your way through the file until you find the XWPFRun of interest, work out what you want the new text to be, and replace it. (A run is a sequence of text with the same formatting)

You should be able to do something like:

XWPFDocument doc = new XWPFDocument(OPCPackage.open("input.docx"));
for (XWPFParagraph p : doc.getParagraphs()) {
    List<XWPFRun> runs = p.getRuns();
    if (runs != null) {
        for (XWPFRun r : runs) {
            String text = r.getText(0);
            if (text != null && text.contains("needle")) {
                text = text.replace("needle", "haystack");
                r.setText(text, 0);
            }
        }
    }
}
for (XWPFTable tbl : doc.getTables()) {
   for (XWPFTableRow row : tbl.getRows()) {
      for (XWPFTableCell cell : row.getTableCells()) {
         for (XWPFParagraph p : cell.getParagraphs()) {
            for (XWPFRun r : p.getRuns()) {
              String text = r.getText(0);
              if (text != null && text.contains("needle")) {
                text = text.replace("needle", "haystack");
                r.setText(text,0);
              }
            }
         }
      }
   }
}
doc.write(new FileOutputStream("output.docx"));