However, this does not mean that you can't make any preparations at all. Before you reply, you can practice and familiarize yourself with your paper many times, so that you can answer the questions clearly. You can make a short outline or memory point to help you recall the key information and points in the defense.
The most important thing is to remain confident and calm. Defense is an opportunity to show your research achievements and abilities, trust your knowledge and preparation, actively answer questions and communicate with examiners.