If you are a user of Opera, you should be getting an update by now. This new release, Opera 12.01, contains numerous bug fixes and stability improvements from the previous big update, Opera 12.
This release contains a true load updates, such as fixing auto complete issues with the address bar, to fixes for HTML exploits. Below is the links for each change log. Most users are using Windows, so this article was mostly based off of the Windows change log.
Windows Change Log | Mac Change Log | Linux Change Log (UNIX)
Your browser should automatically download the update, unless you are on Linux and then you would have to use your package manager/update manager. Otherwise, an alert is in the bottom right of the browser, where the bottom bar is.