test.cs

using (TXTextControl.ServerTextControl tx = new ServerTextControl())
{
  tx.Create();
  tx.Load("template.docx", TXTextControl.StreamType.WordprocessingML);

  foreach (SignatureField field in tx.SignatureFields)
  {
    var signerData = field.SignerData;
    field.SignerData = new SignerData(field.SignerData.Name,
      field.SignerData.Title,
      "Test",
      field.SignerData.ContactInfo,
      "Contract verification");
  }

  tx.Save("template_changed.tx", TXTextControl.StreamType.InternalUnicodeFormat);
}