How to open the text file on the same frame when it runs first time?

Question 1

Okay, lets start here...

public class DemoMenu extends Frame 
{
    Frame frm=new Frame();

You extend Frame but then you create another reference to another Frame as well.

Instead of using DemoMenu, you seem to be using this "other" frm instead...

You're also mixing heavy and lightweight components (AWT/Swing), this is never going to end well. Instead of

Frame, use JFrame
Menu, use JMenu
MenuItem, use JMenuItem

Which lets me introduce you to setDefaultCloseOperation which will allow you to define what action should take place when the use tries to close window via the "x" button...

If you open a stream, you are responsible for closing it (see you file open method).

This should make your code look more like...

import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.File;
import java.io.FileInputStream;
import java.util.Date;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JOptionPane;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class DemoMenu extends JFrame {

//    Frame frm = new Frame();
    JMenuBar bar = new JMenuBar();
    //frm.setJMenuBar(bar);
    JMenu mf = new JMenu("File");
    JMenu me = new JMenu("Edit");
    JMenu mfo = new JMenu("Format");
    JMenu mv = new JMenu("View");
    JMenu mh = new JMenu("Help");
    JMenuItem mf1 = new JMenuItem("New" + "     " + "Ctrl+N");
    JMenuItem mf2 = new JMenuItem("Open..." + "     " + "Ctrl+O");
    JMenuItem mf3 = new JMenuItem("Save" + "     " + "Ctrl+S");
    JMenuItem mf4 = new JMenuItem("Save As...");
    JMenuItem mf5 = new JMenuItem("Page Setup...");
    JMenuItem mf6 = new JMenuItem("Print..." + "     " + "Ctrl+P");
    JMenuItem mf7 = new JMenuItem("Exit");
    JMenuItem me1 = new JMenuItem("Undo" + "     " + "Ctrl+Z");
    JMenuItem me2 = new JMenuItem("Cut" + "     " + "Ctrl+X");
    JMenuItem me3 = new JMenuItem("Copy" + "     " + "Ctrl+C");
    JMenuItem me4 = new JMenuItem("Paste" + "     " + "Ctrl+V");
    JMenuItem me5 = new JMenuItem("Delete" + "     " + "Del");
    JMenuItem me6 = new JMenuItem("Find..." + "     " + "Ctrl+F");
    JMenuItem me7 = new JMenuItem("Find Next" + "     " + "F3");
    JMenuItem me8 = new JMenuItem("Replace..." + "     " + "Ctrl+H");
    JMenuItem me9 = new JMenuItem("Go To" + "     " + "Ctrl+G");
    JMenuItem me10 = new JMenuItem("Select All" + "     " + "Ctrl+A");
    JMenuItem me11 = new JMenuItem("Date/Time" + "     " + "F5");
    JMenuItem mfo1 = new JMenuItem("Word Wrap");
    JMenuItem mfo2 = new JMenuItem("Font...");
    JMenuItem mv1 = new JMenuItem("Status Bar");
    JMenuItem mh1 = new JMenuItem("Help Topics");
    JMenuItem mh2 = new JMenuItem("About Notepad");

    public static void main(String args[]) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                DemoMenu dm = new DemoMenu();
            }
        });

    }

    public DemoMenu() {
        super("United- Notepad");
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setJMenuBar(bar);
        bar.add(mf);
        bar.add(me);
        bar.add(mfo);
        bar.add(mv);
        bar.add(mh);

        mf.add(mf1);
        mf.add(mf2);
        mf.add(mf3);
        mf.add(mf4);
        mf.addSeparator();
        mf.add(mf5);
        mf.add(mf6);
        mf.addSeparator();
        mf.add(mf7);


        mf1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                JOptionPane smd = new JOptionPane();
                smd.showMessageDialog(null, "Press Ok to open new page");

            }
        });

        final JTextArea jta = new JTextArea(10, 20);
        JScrollPane scrl = new JScrollPane(jta);
        add(scrl);

        mf2.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {

                JFileChooser chooser = new JFileChooser();
                int selection = chooser.showOpenDialog(DemoMenu.this);
                if (selection == JFileChooser.APPROVE_OPTION) {

                    JOptionPane.showMessageDialog(null, "so u want to open the file...");
                    File file = chooser.getSelectedFile();
                    FileInputStream fis = null;
                    try {
                        fis = new FileInputStream(file);
                        byte b[] = new byte[fis.available()];
                        fis.read(b);
                        String str = new String(b);
                        jta.setText(str);
                    } catch (Exception f) {
                        f.printStackTrace();
                    } finally {
                        try {
                            fis.close();
                        } catch (Exception exp) {
                        }
                    }
                }
            }
        });

        mf3.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                JFileChooser chooser = new JFileChooser();
                chooser.showSaveDialog(null);
            }
        });


        mf4.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                JFileChooser chooser = new JFileChooser();
                chooser.showDialog(null, "Save As");
            }
        });

        mf7.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                JOptionPane scd = new JOptionPane();
                scd.showConfirmDialog(null, "Are you want to exit");
            }
        });
        me.add(me1);
        me.addSeparator();
        me.add(me2);
        me.add(me3);
        me.add(me4);
        me.add(me5);
        me.addSeparator();
        me.add(me6);
        me.add(me7);
        me.add(me8);
        me.add(me9);
        me.addSeparator();
        me.add(me10);
        me.add(me11);
        me11.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                Date d1 = new Date();
                JOptionPane smddate = new JOptionPane();
                smddate.showMessageDialog(null, d1);

            }
        });

        mfo.add(mfo1);
        mfo.add(mfo2);

        mv.add(mv1);

        mh.add(mh1);
        mh.addSeparator();
        mh.add(mh2);

        setSize(520, 540);
        setVisible(true);
    }
}

Question 2

This happens because your class extend JFrame and from that JFrame you create a new JFrame. Remove the line Frame frm=new Frame(); and use this instead of frm