The OS X El Capitan beta was available for developers shortly after Apple's keynote finished on 8 June, if you are signed up to the Apple Developer Program you will be able it download it now. You can now join the OS X and iOS developer programs for a single fee of $99 (£64) a year, rather than joining the two programs for $99 each per year. Developing for the WatchOS is also included in that fee. To join the Apple Developer Program enroll here.
The great news is that you don't have to be a developer to get your hands on the beta of the new OS X. A public beta is now available for consumers as of 9 July. You can sign up for the Apple Beta Software Program here.
To join the program just sign in with your Apple ID and accept the Apple Beta Software Program Agreement.
Once you are registered for the Apple Beta Software Program you will need to enroll your Mac here in order to be able to get the updates from the Mac App Store.
You will find a download link and you redemption code on that page of Apple's website. Log on to receive it - you won't get an email.
You will also have to re-enroll your Mac if you had previously been a tester for Yosemite.
One you have signed up for the public beta and got your code you can redeem in the App Store. You can then install the software via the App Store just as if it was a normal update.
Before you sign up for that beware that you will be testing pre-release versions of OS X El Capitan (and iOS 9 which is also part of the same program). Running a pre-release version of the software means that you will experience bugs and performance issues, so you might not wish to install El Capitan on your main Mac for fear of losing data, or you might prefer to install is on a secondary partition of the Mac. We'd recommend a full back up before you install it.
What's the difference between the Public vs Developer Preview of El Capitan
The public beta is not the same as the beta being released though the developer program. it is likely that developers will receive more frequent updates including new features not in the public beta.
What's in the latest beta of El Capitan?
The first public beta was the same as Developer Beta 3. That means it at least has fewer bugs and is slightly more stable than Developer Beta 1 was.
Compared to the original developer beta, the public beta included some improvements to the Photos app, including the ability to sort albums by date or title, and the ability to add location to images. In version 3 developers gained access to editing extensions. There was also a change to Two-Factor Authentication.
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