How to navigate to previous fragment using back-bu

I know that similar questions have been asked before, but I can't seem to find exactly what I’m looking for. My problem is that my back button is currently exiting the app. What I’m trying to do is to make it navigate to the previous fragment. Like if you open fragment1 - > fragment2 and then press the back button it would take you back to fragment1.

My current code looks like this.

public class MainActivity extends FragmentActivity {
private DrawerLayout mDrawerLayout;
private ListView mDrawerList;
private ActionBarDrawerToggle mDrawerToggle;

private CharSequence mDrawerTitle;
private CharSequence mTitle;
private String[] mPageTitles;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mTitle = mDrawerTitle = getTitle();
    mPageTitles = getResources().getStringArray(R.array.menu_array);
    mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
    mDrawerList = (ListView) findViewById(R.id.left_drawer);

    // set a custom shadow that overlays the main content when the drawer
    // opens
    mDrawerLayout.setDrawerShadow(R.drawable.drawer_shadow,
            GravityCompat.START);





    // set up the drawer's list view with items and click listener
    mDrawerList.setAdapter(new ArrayAdapter<String>(this,
            R.layout.drawer_list_item, mPageTitles));
    mDrawerList.setOnItemClickListener(new DrawerItemClickListener());

    // enable ActionBar app icon to behave as action to toggle nav drawer
    getActionBar().setDisplayHomeAsUpEnabled(true);
    getActionBar().setHomeButtonEnabled(true);

    // ActionBarDrawerToggle ties together the the proper interactions
    // between the sliding drawer and the action bar app icon
    mDrawerToggle = new ActionBarDrawerToggle(this, /* host Activity */
    mDrawerLayout, /* DrawerLayout object */
    R.drawable.ic_drawer, /* nav drawer image to replace 'Up' caret */
    R.string.drawer_open, /* "open drawer" description for accessibility */
    R.string.drawer_close /* "close drawer" description for accessibility */

    )

    {
        public void onDrawerClosed(View view) {
            getActionBar().setTitle(mTitle);
            invalidateOptionsMenu(); // creates call to
                                        // onPrepareOptionsMenu()
        }

        public void onDrawerOpened(View drawerView) {
            getActionBar().setTitle(mDrawerTitle);
            invalidateOptionsMenu(); // creates call to
                                        // onPrepareOptionsMenu()
        }

    };
    mDrawerLayout.setDrawerListener(mDrawerToggle);

    // Sets the firstpage if no state is found
    if (savedInstanceState == null) {
        selectItem(0);
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.main, menu);

    return super.onCreateOptionsMenu(menu);
}


/* Called whenever we call invalidateOptionsMenu() */
@Override
public boolean onPrepareOptionsMenu(Menu menu) {
    // If the nav drawer is open, hide action items related to the content
    // view
    return super.onPrepareOptionsMenu(menu);
}

// Toogles the drawer menu
@Override
public boolean onOptionsItemSelected(MenuItem item) {
    return mDrawerToggle.onOptionsItemSelected(item);

}

/* The click listner for ListView in the navigation drawer */
private class DrawerItemClickListener implements
        ListView.OnItemClickListener {
    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position,
            long id) {
        selectItem(position);
    }
}

private void selectItem(int position) {
    // update the main content by replacing fragments

    FragmentManager fragmentManager = getFragmentManager();

    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

    switch (position) {
    case 0:
        KontaktInfoFragment kontakt = new KontaktInfoFragment();
        fragmentTransaction.replace(R.id.content_frame, kontakt);
        fragmentTransaction.addToBackStack("Replace");
        fragmentTransaction.commit();

        break;
    case 1:
        OmOssFragment omoss = new OmOssFragment();
        fragmentTransaction.replace(R.id.content_frame, omoss);
        fragmentTransaction.addToBackStack("Replace");
        fragmentTransaction.commit();
        break;
    case 2:
        MainScreenActivity mainscreen = new MainScreenActivity();
        fragmentTransaction.replace(R.id.content_frame, mainscreen);
        fragmentTransaction.addToBackStack("Replace");
        fragmentTransaction.commit();
        break;
    case 3:
        LoginFragment login = new LoginFragment();
        fragmentTransaction.replace(R.id.content_frame, login);
        fragmentTransaction.addToBackStack("Replace");
        fragmentTransaction.commit();
        break;
    case 4:
        ContactFormFragment form = new ContactFormFragment();
        fragmentTransaction.replace(R.id.content_frame, form);
        fragmentTransaction.addToBackStack("Replace");
        fragmentTransaction.commit();
        break;  

    }

    // update selected item and title, then close the drawer
    mDrawerList.setItemChecked(position, true);
    setTitle(mPageTitles[position]);
    mDrawerLayout.closeDrawer(mDrawerList);
}

@Override
public void setTitle(CharSequence title) {
    mTitle = title;
    getActionBar().setTitle(mTitle);
}

}

When you press back button inside your fragment onBackPressed() method of your activity will be called if you have declared that..So handling back button for fragments within navigation drawer can be one in this way..

MainActvity

     public static boolean isMainActivityShown ;
     public static boolean isFragment1Shown=false ;
     public static boolean isFragment2Shown=false ;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        isMainActivityShown=true  //inside onCreate method put isMainActivityShown true
. 
. 
.
}
                 Fragment currentFragment = new Fragment1();
                  isMainActivityShown=false;   //when moving to fragment1 
                isFragment1Shown=true;
               frgManager = getFragmentManager();
               frgManager.beginTransaction().replace(R.id.content_frame, currentFragment)
                                        .commit();
     @Override
        public void onBackPressed() {

            if(isMainActivityShown)
            {
                finish();
            }
            else if(isFragment1Shown)
            {
               //write the code to handle back button when you are in Fragment1
            }
           else if(isFragment2Shown)
            {  //When you are in Fragment 2 pressing back button will move to fragment1
                Fragment currentFragment = new Fragment1();
                isFragment2Shown=false;
    isFragment1Shown=true;

                FragmentManager frgManager;
                frgManager = getFragmentManager();
                frgManager.beginTransaction().replace(R.id.content_frame, currentFragment)
                        .commit();
            }

            }

Fragment1

Fragment currentFragment = new Fragment2();

            MainActivity.isFragment1Shown=false;
MainActivity.isFragment2Shown=true;
                            frgManager = getFragmentManager();
                            frgManager.beginTransaction().replace(R.id.content_frame, currentFragment)
                                    .commit();