android navigate to another fragment

Intent i=new Intent(MainActivity.this,SecondActivity.class);
startActivity(i);

or replace the fragment

Fragment fragment = new tasks();
FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.replace(R.id.content_frame, fragment);
fragmentTransaction.addToBackStack(null);
fragmentTransaction.commit();