OS X Mavericks is on the way. For now, however, Apple has released OS X Mountain Lion 10.8.5. It’s recommended for all OS X Mountain Lion users.
Apple says you should back up your system before installation. Then choose Apple menu () > Software Update to check for the latest Apple software via the Mac App Store (including this update).
You can also download the manual update installer. This is a useful option when you need to update multiple computers but only want to download the update once. These versions of the standalone installers are available from Apple Support Downloads
According to Apple, OS X Mountain Lion 10.8.5:
° Improves the stability, compatibility, and security of your Mac;
° Fixes an issue that may prevent Mail from displaying messages;
° Improves AFP file transfer performance over 802.11ac Wi-Fi;
° Resolves an issue that may prevent a screen saver from starting automatically;
° Improves Xsan reliability;
° Improves reliability when transferring large files over Ethernet;
° Improves performance when authenticating to an Open Directory server;
° Addresses an issue that prevented a smart card from unlocking preference panes in System Preferences;
° Contains the improvements included in MacBook Air (Mid 2013) Software Update 1.0.
Apple has also released security updates for Mac OS X 10.6 Snow Leopard and OS X Lion, both the consumer and server versions of the software.