This is pretty easy to do just follow these steps.
1.“Please note that you are about to install beta software. Always back up your data and files before installing beta versions of OS X. If you have multiple Macs, we recommend installing OS X Yosemite Beta on your secondary computer. Backing up files on a Mac is easy with Time Machine, the built-in backup utility in OS X.”
2. “Click on the “Download Beta Access Utility” button below. This will download the YosemiteBetaAccessUtility.dmg. Open the disk image, then double-click the package to run the installer and enroll your Mac for Beta Software updates.”
Once you have installed the .dmg file you should get a screen that looks like this.
3. “The Mac App Store will automatically launch and display the Updates pane. There may not be any beta updates available at this time. If there are, click the Update button to install the available updates. If not, you will be notified by the Mac App Store when they are available.”