I know this is an old post but i am sure that this might help someone down the road.
You can simply get the headerView element of the Navigation view by doing this:
NavigationView mView = ( NavigationView ) findViewById( R.id.nav_view );
if( mView != null ){
LinearLayout mParent = ( LinearLayout ) mView.getHeaderView( 0 );
if( mParent != null ){
// Set your values to the image and text view by declaring and setting as you need to here.
}
}
I hope that this helps someone.