The Extra theme displays a “Related Posts” section on the single blog page at the bottom under author profile. If you’d like to change the text of “Related Posts” to something else (e.g. “Popular Posts”), you can do so with the following PHP code:

add_filter('gettext', 'dbeb_changeRelatedPostsText', 20, 5);

if (!function_exists('dbeb_changeRelatedPostsText')) {
	function dbeb_changeRelatedPostsText($translation, $text, $domain ) {
		if (is_single() && $domain === 'extra' && $text === 'Related Posts') {
			return 'Popular Posts'; 
		return $translation;

Just substitute “Popular Posts” with your desired text in the above code.

As this is PHP code, you can add it to the functions.php of a child theme, or using a plugin such as Code Snippets. Please take a backup prior to adding PHP code to your site, so that you can restore the original version if necessary.