Wordpress Core Development Released WordPress 3.6 Codename “Smith” is now available for download or if you are running 3.8.3 then you can update that from

Download

or Update
Dashboard >> Updates

Whats New

1. Updated Media upload edit capabilities
2. Live widgets preview
3. New Theme Browser

For Developers

1. Now Galleries with HTML5 markup
2. New Functions

Identify a hook in progress with doing_action() and doing_filter() , and manipulate custom image sizes with has_image_size() and remove_image_size() .

3 . Improved compatibility for PHP 5.5 and MySQL 5.6

See Full Release Notes : http://wordpress.org/news/2014/04/smith/