Earlier today Apple opened up its public beta program for the upcoming OS X Yosemite release allowing the public to download the beta build from the Mac App Store. Those who signed up for the program received a code through Apple’s Beta Program website, but a number of users reported experiencing an error that said their code had already been redeemed. If you’re still having trouble, here’s the fix from Apple: Refresh the beta program redemption page when signed in to generate a new code (here). That’s it. The new code should work when entered into the Mac App Store.
Apple doesn’t plan to update the public beta build of the OS as much as the version available in its Developer Program, but testers will be able to update to the final public release of Yosemite through the Mac App Store when it’s released this fall. Info on the release, signing up, and known issues are available here.