What you'll need...

OS X 10.10 Developer Preview -- Developers can grab the preview through the Dev Center. But if you are not a developer, you could sign up for the new OS X Beta Program and wait for Apple to finally send out invites, or you can download it right here by yourself.

Storage Space -- If you’re having low storage space on hard drive, you’ll need to free up to 20GB or more before beginning the partitioning and installation process.


Types of OS X Yosemite Installation Options

Yosemite supports the usual installation options; clean install and upgrade install. The clean install option has some variants, such as installing on a non-startup drive or on your current startup drive.

Clean Install: Clean install process gets its name from what it will do to your designated target drive, usually your startup drive. It will erase all data completely on the target drive and replace it with the new OS X Yosemite and the default apps that come with it. Making clean install option as your choice will leave your Mac in a similar condition to the day you got it; no user accounts, no user data, and a setup wizard to walk through to create your first administrator account.

A clean install really is for starting from scratch. So, before you decide to make clean install option as your choice, be sure to back up all of your data in order for you not to loose them.

Upgrade Install: Upgrade install will remove the existing version of OS X completely from the startup drive. It will update all applications that Apple includes with the OS, such as Safari and Mail and it will also update all needed system files. An upgrade install won't make changes to your user data; as a result, your user accounts and any data associated with them will remain the same. When you first run an app, such as Mail, it's likely that your mail data will be updated to work with the new version of Mail included with Yosemite. This same user data upgrade may occur with multiple apps. For this reason, you should not expect to be able to go back to the previous version of the OS if you decide you don't like OS X Yosemite, at least not without using the backup you made before you installed it.


How to Upgrade Install OS X Yosemite on Your Mac

Let's Get Started
The first step in installing Yosemite is to check your Mac's startup drive for any problems, including repairing permissions. You can do this by using the instructions in our guide:

Using Disk Utility to Repair Hard Drives and Disk Permissions

When you're done, come back here and follow the upgrade install process.

OS X Yosemite is available from the Mac App Store, and is a free upgrade from OS X Snow Leopard (10.6.x) or later. If you're running a version of OS X older than 10.6.x, you'll need to first purchase Snow Leopard and then install it on your Mac.
Download OS X Yosemite

Launch the Mac App Store by clicking its icon in the Dock.

You'll find OS X Yosemite in the right-hand All Categories sidebar, under the Apple Apps category. But, if you signed up for the OS X Yosemite public beta and received a beta access code from Apple, you'll find the download by clicking the Purchases tab at the top of the Mac App Store window.

Select the OS X Yosemite app and click Download button (the download is in excess of 5 GB, so it will take a bit of time).

Once the download process is complete, you're ready to start the installation process.
Upgrade Install OS X Yosemite

1. Install OS X Yosemite, the installer usually starts up automatically after the download is complete; if it didn't start up, simply double-click the Install OS X Yosemite file.

2. When the Install OS X app opens, click the Continue button to proceed the upgrade.

3. Click the Agree button of Yosemite license agreement to proceed.
4. A small sheet will appear, asking you to confirm that you have actually read the license agreement. Click the Agree button.
5. You'll be presented with your Mac's startup drive as the install destination for OS X Yosemite. If this is correct, click the Install button. You can also select the Show all Disks button to allow you to select a different drive to install on. If you don't wish to overwrite your startup drive with the new OS, select Quit Install OS X from the Install OS X menu.

6. Enter your administrator password and click OK.
7. The installer will begin by writing needed files to the startup drive; this process can take a few minutes. When it's complete, your Mac will restart.
8. After the restart, your Mac will display a gray screen with a progress bar for a short period of time. Eventually, the display will change to show an install window, with a progress bar and a time estimate.Once the progress bar completes, your Mac will restart again, and you'll be taken to the login screen.
9. The OS X Yosemite has been installed and you're ready to start the setup process, where you configure the OS to meet your personal preferences.

Read more: http://trendymediablog.com/2014/08/how-to-upgrade-install-os-x-yosemite-on-your-mac/