public class FragActivity extends FragmentActivity {

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

        ViewPager pager = (ViewPager) findViewById(R.id.pager);
        PagerAdapter adapter = new FFNPagerAdapter(getSupportFragmentManager());

        pager.setAdapter(adapter);
        pager.setOffscreenPageLimit(3);
        tabs.setViewPager(pager);
    }
}

public class PagerAdapter extends FragmentPagerAdapter implements TextAndIconTabProvider {
    private final String[] TITLES = { "FIRST", "SECOND", "THIRD", "FORTH" };

    public PagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        Fragment frag = new Fragment();
        switch (position) {
        case 1:
            frag = new FirstFrag();
            break;
        case 2:
            frag = new SecondFrag();
            break;
        case 3:
            frag = new ThirdFrag();
            break;
        case 4:
            frag = new ForthFrag();
            break;

        }

        return frag;
    }

    @Override
    public int getCount() {
        // TODO Auto-generated method stub
        return TITLES.length;
    }
}

public class FirstFrag extends Fragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.firstfrag, container, false);
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onActivityCreated(savedInstanceState);

        Button btn1 = (Button) getActivity().findViewById(R.id.btn1);
        btn1.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                // what should I put in this block??????
                Fragment fragment = new FirstFragSub1();
                FragmentTransaction transaction = getFragmentManager().beginTransaction();
                transaction.replace(R.id.pager, fragment); <<<<<<<<<------------------???????
                transaction.commitAllowingStateLoss();
            }
        });
    }
}