Mac OS X will prompt you for critical updates when they are available. However, it also recommended to regularly run Software Update to make sure no updates are missed.
To update software in OS X, open the APPLE MENU and select SOFTWARE UPDATE. This is also available under SYSTEM PREFERENCES.
Software Update will automatically launch and check for updates.
If there are any updates found, you will see them listed and be prompted to install them. You may be prompted to enter your computer password as part of the process. Once the updates are installed, you may be prompted to restart.
If you run Software Update and no updates are found, you will see the message, "Your software is up to date. Please try again later."