force http canonical tag on https pages

You can change it using following code, add it in your theme function.php or in plugin.

remove_action ( 'wp_head' , 'rel_canonical' ) ;
add_action ( 'wp_head' , 'my_rel_canonical' ) ;

function my_rel_canonical () {
    ob_start () ;
    rel_canonical () ;
    $rel_content = ob_get_contents () ;
    ob_end_clean () ;
    echo str_replace ( "https:" , "http:" , $rel_content ) ;
}