how to modify feed templates

this is a good start for that:
https://yoast.com/custom-rss-feeds-wordpress/

Thanks @rahul Sonar,

That’s a good trick.

However, since what I want is quite simple: add the post excerpt before rss post content(since I use excerpt to provide a summary and introduction of post content so there’s no duplicate between excerpt and content), I think maybe some functions would do the job, something like this: https://www.wpbeginner.com/wp-tutorials/how-to-add-post-thumbnail-to-your-wordpress-rss-feeds/

But my code seems not working:

function rss_with_excerpt($content){
	global $post;
	if(has_excerpt($post->ID)){
	$content = get_the_excerpt() . get_the_content();
}
	return $content;
}
add_filter('the_content_feed','rss_with_excerpt');