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, it can be added to the theme by copying it into your theme or child theme's functions.php file. Please note that if you copy it into Extra's own functions.php file, it will be overwritten when the theme is updated. For that reason it's advisable to add it via a child theme. Please also be aware that mistakes made when altering the PHP code of a WordPress site can cause the site to become inaccessible. Please take a backup of your theme / child theme files prior to changing them so that you can restore the original version if necessary.