BuddyPress.org

Search for a plugin and then you will need to replace the navigation links in your header.php file with the code to call the plugins drop down menu. You can also add support for wp menus and then do the css or javascript coding to make it drop down.

http://www.wpbeginner.com/wp-themes/how-to-add-custom-navigation-menus-in-wordpress-3-0-themes/