Consider a clean installation on a separate volume or partition: With every OS update — mobile or desktop — there are people who make the switch and then regret doing so. It may be Yosemite's new "flatter" look that puts them off or that Yosemite's performance on their Mac isn't everything they'd hoped it would be. By installing Yosemite on an external drive or a partition on your startup volume you have the option to experience it before making a complete commitment. If you aren't satisfied with it, it's a simple matter to boot back into your Mavericks volume. If you like what you see, go right ahead and install it on your regular startup volume.
As were the last few versions of OS X, Yosemite is available from the Mac App Store. To get your copy select App Store from the Apple menu. For the first several days that Yosemite is available, Apple will display a banner promoting it. (If you're reading this months after its release, scroll down the page and look under the Free heading along the right side of the window. OS X Yosemite likely appears within the top 10 free downloads.)
In either case, you're taken to the OS X Yosemite page where you can download Yosemite for free. Click the appropriate button to download it and then enter your Apple ID and password when prompted. The installer will start downloading and be saved to the Applications folder at the root level of your hard drive.
This download exceeds 5GB so if you have a slow broadband connection, be prepared to wait awhile. (If your connection is really slow consider starting the download before you leave the office or go to bed. It should be ready for you the next morning.) Note that if you're downloading it on the first day of release, it may take longer to get to you than it would a a day or two later due to the large number of people trying to get their copy.
Saving a copy of the installer
As part of the installation process the Yosemite installer deletes itself once it finishes its job. This isn't entirely distressing as you can always download another copy should you need it.
However, if you have more than one Mac that you'd like to upgrade to Yosemite or you prefer to have the installer around should you need to install a fresh copy at a later date without waiting for it to download, you must make a copy of the installer outside of the Applications folder. Do so and it won't be deleted after the installation.
If you attempt to drag it out of the Applications folder, an alias of it will be created rather than a copy. To make the copy, hold down the Option (Alt) key and drag it to another destination.
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