Spinner sp = (Spinner) getLayoutInflater().inflate(R.layout.nav_header_xyz,null).findViewById(R.id.MySpinner);
    String[] a = new String[]{"abc","xyz"};
    ArrayAdapter<String> aa =new ArrayAdapter<String>(this,R.layout.support_simple_spinner_dropdown_item,a);
    aa.notifyDataSetChanged();
    System.out.println(sp);
    sp.setAdapter(aa);

    navigationView.invalidate();

Spinner sp = (Spinner)((NavigationView) findViewById(R.id.nav_view)).findViewById(R.id.MySpinner);
    String[] a = new String[]{"abc","xyz"};
    ArrayAdapter<String> aa =new ArrayAdapter<String>(this,R.layout.support_simple_spinner_dropdown_item,a);
    aa.notifyDataSetChanged();
    System.out.println(sp);
    sp.setAdapter(aa);

    navigationView.invalidate();