How to change RSS output

I wish to modify the default layout of RSS feeds:

– Make visible only the title of the post

or

– tweak the post’s body

In the documentation it is stated that WP uses some templates to do so. Unfortunatelly editing them shows the following:

<?php
/**
 * Redirects to the RSS2 feed
 * This file is deprecated and only exists for backwards compatibility
 *
 * @package WordPress
 */

require( './wp-load.php' );
wp_redirect( get_bloginfo( 'rss2_url' ), 301 );

?>

That is it looks like there’s no way to customize the output.
How do I achieve this?