On Thursday, fall will come early for hundreds of thousands of Mac users when Apple releases its first public beta of OS X Yosemite. The public-beta program, announced during Apple’s annual developer conference in June, lets regular users download and test pre-release versions of OS X. Apple says the first million users to sign up at the OS X Beta Program website will be able to test Yosemite before the OS is released to the general public in the fall. […]
When Yosemite is finished, users will be upgraded to the final version automatically, also via the Software Update feature within the App Store app.
Just as a reminder, OS X Yosemite is still software that is under development, so apps and services may not work as expected some (or all) of the time. It’s always wise to back up your Mac before installing, and you should seriously consider installing Yosemite on a Mac that you don’t use for day-to-day operations just in case something goes wrong.