Android : switch between fragments without 'FragmentPagerAdapter'

I am trying to switch between fragments on my activity. I was reading this tutorial, but my case is a bit different since I don't want/can't use the 'FragmentPagerAdapter', instead, I want that a button that is pressed on activity will switch between 2 fragments.

My activity layout consist of a Button and ViewPager. in Addition I have got Fragment1 and Fragment2. How can I switch between this fragments using OnClick method?

My Layout:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <Button
     android:text="Switch it"
     android:onClick="switchFragment"/>

    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    </android.support.v4.view.ViewPager>

</LinearLayout>

and my activity:

public class MainHeaderFragment2 extends ActionBarActivity {    
    ...
    public void switchFragment(View view){
         Fragment fragment
         if(checkSomething())
           fragment = new Fragment1();
         else
           fragment = new Fragment2();
        ViewPager viewPager = (ViewPager) findViewById(R.id.pager);
        // now need to put the selected fragment in ViewPager somehow. 
        // How? that is my question
    }
}