getSupportFragmentManager()
            .beginTransaction()
            .addToBackStack("ToDoFragment")
            .replace(R.id.content_frame, fragmentR, fragmentR.getClass().getSimpleName())
            .commit();

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);

    // If navigation drawer is open, close it
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
        setDrawerState(true);
        displayMenu(true);
    }
}

getFragmentManager().beginTransaction()
                .add("ToDoFragment")
                .replace(R.id.content_frame, new SettingsFragment()).commit();