The OS X Mountain Lion v10.8.5 Update is recommended for all OS X Mountain Lion users.

Updating your system

1. You should back up your system before installation. To do this you can use Time Machine.
2. Do not interrupt the installation process once you have started to update your system.
3. You may experience unexpected results if you have third-party system software modifications installed, or if you've modified the operating system through other means.
4. Choose Apple menu () > Software Update to check for the latest Apple software via the Mac App Store, including this update.
5. Other software updates available for your computer may appear, which you should install. Note that an update's size may vary from computer to computer when installed using Software Update. Also, some updates must be installed prior to others.

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. 

About the update

The OS X Mountain Lion v10.8.5 Update is recommended for all OS X Mountain Lion users. It improves the stability, compatibility, and security of your Mac. This update:

• 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