How do I edit my default RSS Feed?

Question

If you want to change the content shown in a feed, use the the_content_feed filter:

add_filter( 'the_content_feed', function ( $content, $feed_type ) {
    $post_id = get_the_ID();

    if ( $feed_type === 'atom' )
         $content="something else";
    else
         $content = wp_trim_words( $content, 30 );

    return $content;
}, 10, 2 );

The above is purely an example of some of the (bizarre) things you might want to do.