function featuredtoRSS($content) {
global $post;
if ( has_post_thumbnail( $post->ID ) ){
$content = '<div>' . get_the_post_thumbnail( $post->ID, 'medium', array( 'style' => 'margin-bottom: 15px;' ) ) . '</div>' . $content;
}
return $content;
}

add_filter('the_excerpt_rss', 'featuredtoRSS');
add_filter('the_content_feed', 'featuredtoRSS');

add the above code to your functions.php, its located in your theme folder,
how to navigate there;
Log on to your cPanel, file manager, root folder (usually "public_html"), wp-content, themes, functions.php

note: this method is best is you are using child theme, 

there are quite few plugins out there which can be used to add functions to fucntions.php without going to your hosing or cPlanel.

I recommend  to "Code Snippet" it worked fine for me on multiple websites