Although it is out of the Scope of Technical Support for Blizzard to assist, you can search on your own for a program that will tell you what type of motherboard you have, then visit the manufacturer’s website for the update.
You can find the latest Combo Update for your operating system located on Apple's Support Download page.
When an older OS has no more incremental updates, you will need upgrade to a new OS to obtain newer drivers and updates.
Other components control the creation of raster (2D) graphics (Direct Draw / Direct2D), fonts (Direct Write) or web animations (Direct Animation).
Sound playback and processing also comes to life thanks to APIs like Direct Sound or Direct Music (XAudio2 and XACT3).
However, Microsoft has been known to release updated versions of Direct X and installing the latest updates may be the fix to a Direct X problem you're having or may give performance increases in your games and graphics programs.
Out-of-date drivers and operating systems can cause a variety of issues, and are one of the first things to check when a game is having problems.
You can try rolling back your system to before your last update, or you can try to repair your Direct X files.
All modern Windows operating systems include Direct X by default so you shouldn't ever need to "install" Direct X as a software program, per se.
Newer Blizzard titles require a recent version of OS X to run properly.