June 4, 2014
Not counting the supposed download files that can circulate on the Internet, and that we do not recommend, the only way to install Yosemite without being a developer is to sign up for the new Help shape OS X Yosemite beta program for the general public. Sign up for our Beta Program. To do this, simply enter this page and register with a valid Apple ID. The program is limited to one million users, so if you are very sure you want to enter, you should register quickly.
The bad news is that joining the program does not mean that you start downloading Yosemite immediately. That will come in a few weeks, when registrants start to receive an email with the download link. Also, although OS X Yosemite Beta is free, Apple expects people who request it to report bugs they find. The company already warns that there are many features that are not yet operational.
via How to install OS X Yosemite before anyone else, and without being a developer.
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