If you are using a Macintosh computer with Mac OS 8.6 through Mac OS 9 , follow the steps in "QuickTime: How to Uninstall QuickTime on a Macintosh" to remove QuickTime 6.
Once you have successfully uninstalled QuickTime 6 you should reinstall QuickTime 5, which is available from the Apple Software Downloads.
If you are using a Macintosh computer with Mac OS X 10.1 through 10.1.5, follow the steps in "QuickTime 5 Reinstaller for Mac OS X: Information and Download" to remove QuickTime 6 and reinstall QuickTime 5.
If you are using a Windows-compatible computer follow the steps in "QuickTime: How to Uninstall QuickTime on a Windows-Compatible Computer" to remove QuickTime 6. When ready to install, see "QuickTime: How to Install on Windows"
Once you have successfully uninstalled QuickTime 6 you should reinstall QuickTime 5, which is available from the Apple Software Downloads.