Hi vasiliy,

You need to modify the wpForo_Widget_recent_topics and wpForo_Widget_recent_replies classes, it can be found in the wp-content/plugins/wpforo/wpf-includes/functions-template.php file. 

However, you'll need to keep these changes, in a case to not lose it during the upgrade process. This is a core file, this is not a template file to customize.

Thanks