How can I change my assigned user role in WordPress 3.5.1?

Either use wp_update_user

wp_update_user(
    array (
        'ID' => 1, // replace by your user ID
        'role' => 'editor'
    )
) ;

or the set_role method of the WP_User class.

$you = new WP_User( 1 ); // again, replace parameter by your user ID
$you->set_role( 'editor' );