Here's how you can download the free OS X Mountain Lion 10.8.3 update from Apple.
1. Go to the OS X Mountain Lion 10.8.3 update page.
2. Click the download link on the right.
3. Launch the installer to upgrade your Mac.
4. You can also upgrade via the Mac app store.
Here's a full description:
"This update is recommended for all OS X Mountain Lion users and includes features and fixes that improve the stability, compatibility, and security of your Mac, including:
The ability to redeem iTunes gift cards in the Mac App Store using your Mac's built-in camera
Boot Camp support for installing Windows 8
Boot Camp support for Macs with a 3 TB hard drive
A fix for an issue that could cause a file URL to quit apps unexpectedly
A fix for an issue that may cause Logic Pro to become unresponsive when using certain plug-ins
A fix for an issue that may cause audio to stutter on 2011 iMacs
A fix for an issue in Contacts that may cause cards to print out of order
A fix for an issue that may cause the desktop picture to change after logging out or restarting
A fix for an issue in Messages that may cause messages to appear out of order after waking from sleep
A fix for an issue that may cause the screen to display incorrectly after waking from sleep
Improves compatibility with IMAP servers in the Notes app
Allows the Slideshow screen saver to display photos located in a subfolder
A fix for an issue in Contacts that may cause addresses to print in the wrong location
Reliability improvements when using a Microsoft Exchange account in Mail
Xsan reliability improvements
A fix for an issue that could cause Active Directory accounts to be locked out after accessing the Security & Privacy pane in System Preferences
A fix for an issue that could cause link aggregation to not complete after a restart
A fix for an issue that could cause delays when logging into an Active Directory account on high latency networks
Safari 6.0.3 is included in the OS X Mountain Lion v10.8.3 Update and contains fixes that improve performance, stability, and security, including:
Improves scrolling on facebook.com
Improves scrolling while zoomed in on a webpage
Improves performance on webpages with plug-in content
A fix for an issue that could cause the inaccurate appearance of an alert that bookmarks can't be changed
A fix for an issue that could cause duplicate bookmarks to appear on an iOS device after editing bookmarks with Safari in OS X
A fix for an issue that permitted users to access unfiltered search results when searching from google.com when Parental Controls are enabled
A fix for an issue that could prevent Safari from restoring the last position on a webpage a user navigated back to"