In your functions.php file add in the following code

// Customize the content of the custom RSS feed
function custom_rss_item($content) {
    global $post;

    // Build the custom content for each post item
    // Get the first 500 characters of the post content
	$custom_content .= '<p>' . esc_html(get_the_excerpt()) . '</p>';
	$custom_content .= '<p> </p>';
    $custom_content .= '<p>' . esc_html(substr(strip_tags(get_the_content()), 0, 500)) . '...</p>';
	$custom_content .= '<p> </p>';
	$custom_content .= '<p><a href="' . esc_url(get_permalink()) . '" target="_blank">Read The Full Article Herek</a></p>';

    // Combine with existing content
	$content = $custom_content;

    return $content;
}
add_filter('the_excerpt_rss', 'custom_rss_item');
add_filter('the_content_feed', 'custom_rss_item');