As you known Mac.

Step 1. Backup

Before you do anything regarding installation it is always worth backing up your hard drive. Plug in your Time Machine drive, or any other backup source and have a full copy backup of all your files and folders. Although the installation process will probably not screw anything up, it is always worth having a copy. You should be backing up your data daily in the event of a problem, so this shouldn’t be much of a problem.

Step 2. Download

The first proper stage is to download the app. Open the AppStore, and select the buy now button. The upgrade costs $19.99, which is very good for a large update like this. Once you have purchased the app, it will slowly download. This is by far the longest step of the process.

Step 3. Install

Once the download has completed the installer will run. It will ask you to choose where you want to install the OS. If you have multiple installation locations, for more than one operating system the install will ask where you want to install it. For the most part this will be Macintosh HD.

Sep 4. Restart

The installation process will then restart. Something rather uncommon for my Mac.

Step 5. Install Some More

When your Mac restarts it will go into a longer install process. Just sit back and relax and let the installer do it thing.

Enjoy! 

Via Mactricksandtips