Use setText(str) method of JLabel to dynamically change text displayed. In actionPerform of button write this:
jLabel.setText("new Value");
A simple demo code will be:
JFrame frame = new JFrame("Demo");
frame.setLayout(new BorderLayout());
frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
frame.setSize(250,100);
final JLabel label = new JLabel("flag");
JButton button = new JButton("Change flag");
button.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent arg0) {
label.setText("new value");
}
});
frame.add(label, BorderLayout.NORTH);
frame.add(button, BorderLayout.CENTER);
frame.setVisible(true);