QuickTime is pretty heavily embedded in the OS and is extremely difficult to downgrade. Sometimes if there's a troubled update Apple will release a specialised downgrade 'patch' but this isn't very common and as far as I know, unavailable for QuickTime 7.1.3. 😱
The easiest way to downgrade QuickTime is to reinstall the OS from scratch, however this isn't really very practical and if your machine already came with 7.1.3 then it won't do any good anyway. Perhaps there's a better solution to your problem. What exactly is wrong with 7.1.3? 🙂