Filter

Rename Registered Post Types

add_filter( 'register_post_type_args', 'rename_registered_post_types', 999, 2 );

function rename_registered_post_types( $args, $post_type ){
    if( 'post' == $post_type ){//your post type to rename
        $args['rewrite']['slug'] = 'your_custom_slug';
    }
    return $args;
}