download link:

You must uninstall SP3 RC 1 then run the installer (only applies if you have RC 1 installed otherwise just run the installer and go to windows update). It will add a registry key to your system. Then go to windows update to get the RC 2 version of SP 3. It will not work on systems with RC 1 installed hence why you need to uninstall it first.
This was released to private testers a few weeks ago and is now in our hands to play with. Kudos to Microsoft for getting us release candidates way sooner than I expected. Always remember this is not a final product and has a chance of messing up your windows installation.