There are two ways to change the title of JFrame:

First, you can give a title to your window when creating it in the constructor:

JFrame frame = new JFrame("Title");

Second, once you have created the instance of the JFrame object, you can call the method JFrame.setTitle:

JFrame frame = new JFrame(); frame.setTitle("title");

Example:

To help you I give you this code from the modification of the title of JFrame:

import java.awt.Dimension; 
import javax.swing.JFrame; 

public class Test extends JFrame{
 
 public static void main(String[] args) {
 
 JFrame jframe = new JFrame(); 
 jframe.setTitle("my title"); 
 jframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
 jframe.setLocationRelativeTo(null); 
 jframe.pack(); 
 jframe.setVisible(true); 
 }
}