There's no official way to do this, but there are unofficial methods, which are only recommended if you're happy to mess about in computers' file systems.
When you installed 8.2RC you actually overwrote 8.1, so it can only be recovered if you download it from here:
You then need to extract the plugin from the download, which is harder than it sounds because the files are renamed. I won't go any further for now, in case you're not interested.
Please re-install Ps CS6 to get back to ACR 8.0, Then use Help->Update to update to 8.1.
You can find the Ps CS6 download from the following link.
Please follow the instructions as mentioned and re-install Ps CS6.
Manually replacing files would result in failure of ACR updates in future.
In my experience RCs have been stable, mostly aimed at including new camera models. so I personally wouldn't go back to 8.1. Also, 8.2 RC has been around a while, and final 8.2 should be out soon, and then one can use the usual update methodology.
If you have some issue with 8.2 RC, then you should so state.
I agree with Richard. Unless you have some complaint, I would just keep using the RC until the final release. I think that is going to be next week. I have been using the release candidate for both Lightroom and Camera Raw and haven't had any problems with either one.