If you are unable to change your default homepage under Settings > Reading after installing the demo content, just go to Appearance > Editor > functions.php and scroll towards the bottom of the page until you see the section below. Simply delete this entire section and save the page. You should then be able to change the homepage to whatever you’d like!
//======================================================================
// Change Home Page Settings
//======================================================================
$homepage = get_page_by_title( 'Home' );
if ( $homepage )
{
update_option( 'page_on_front', $homepage->ID );
update_option( 'show_on_front', 'page' );
}