What's included > About the OS X Mountain Lion v10.8.3 Update
The file is 793.69 MB, which will take awhile to download, depending on your internet service download speed.
If you have problems downloading the update, disable anti virus software and turn off the Firewall in System Preferences > Security & Privacy.
In order for the update to be effective, you must restart your Mac. It's ok if your Mac restarts itself more than one time.
DO NOT INTERUPT THE DOWNLOAD AND INSTALL PROCESS.