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If you're anything of a video game fan and you have a Windows computer, you've probably got some version of DirectX installed on your computer. The program often comes bundled with Windows games and is essentially a bunch of libraries that allow computers to make the most of their audio-visual capabilities.
These libraries are not necessarily consistent from version to version. In order to keep your computer up to date, you will need to regularly install the latest version of DirectX. The versions are generally backwards compatible. In other words, if you have a program that needs DirectX 9.0c and you have DirectX 11 installed, you shouldn't need to reinstall DirectX 9.0c in order to get the program to work properly.
However, failing to update DirectX does not cause any sort of security risk. The program allows your computer to get more from its audio and video cards, but it is not considered an essential or critical application from either a security or a computer stability standpoint. You cannot damage your computer by not updating the software, although some video games might not work properly if you regularly avoid new DirectX updates.
Microsoft offers DirectX as a free download. Many games and audio/video programs also come with DirectX because they need the program's libraries in order to work properly. To install the file, you typically just double-click an executable and wait for the program to finish installing. DirectX usually installs itself completely in less than five minutes and does not require a system reboot.
DirectX is ultimately an important program if you use your computer for audio-intensive or video-intensive tasks on a regular basis. If you rarely use your computer for these types of tasks, however, you can probably wait to install the program until you have a clear reason to do so.