In this iText 5 example, we'll change the properties and the value of a text field:
public void manipulatePdf(String src, String dest) throws DocumentException, IOException {
PdfReader reader = new PdfReader(src);
PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(dest));
AcroFields fields = stamper.getAcroFields();
fields.setFieldProperty("text", "textcolor", BaseColor.BLUE, null);
fields.setFieldProperty("text", "bordercolor", BaseColor.RED, null);
fields.setFieldProperty("text", "fontsize", 14, null);
fields.setField("text", "Field Text");
stamper.close();
reader.close();
}
Source: developers.itextpdf.com