Keeping your computer up to date is the single most important task for protecting your computer. By default, Ubuntu will alert you when updates are available for your system, either through Update Manager for desktop environments or via the message of the day (motd) on servers. Security updates for Ubuntu are announced via Ubuntu Security Notices (USNs).
By default, users are notified daily for security updates and weekly for non-security updates. How and when Ubuntu alerts you can be configured in Update Manager. You can access Update Manager anytime by pressing 'Alt+F2', entering update-manager and pressing Enter. Its settings can be adjusted by pressing the 'Settings' button.
Once Update Manager is open, you can review and select pending updates as well as check for new updates. Simply press the 'Install Updates' button to upgrade the selected packages to the updated version.
In Ubuntu 10.04 LTS and later, Ubuntu will also alert you upon console or remote login by default. You can check for and apply new updates with:
$ sudo apt-get update $ sudo apt-get upgrade
Under some circumstances running the above will not apply all updates, which can happen when new packages provided by the update are to be installed. If this happens, simply run sudo apt-get dist-upgrade, review what apt is going to do, and confirm that you want to apply the updates.