To reinstall DirectX, either insert the CD that has the DirectX version you want to reinstall and search for the appropriate file, or locate the DirectX file you've recently downloaded. Once found, double-click the file named "dxsetup.exe" and choose the option to reinstall.
We do not recommend that you attempt to install an older version of DirectX over a newer version, unless the older version is completely uninstalled.
All new games with DirectX support also include a DirectX setup on them. Running the DirectX setup or installing the game also installs DirectX.