Here’s an overview of how to install a major upgrade to OS X. For example, if you would like to upgrade from OS X Yosemite (aka OS X 10.10) to OS X El Capitan (aka OS X 10.11) then you could follow the instructions below. (These instructions assume you have at least OS X 10.6.8 or higher on your Mac.)
Stage 2 – Run the installer to actually install the OS X upgrade
Each stage takes bout 45 minutes, but these are just estimates. They’ll vary based on the speed of your Mac. Before you install a major upgrade to your operating system, please make sure that you have a full Time Machine backup of your Mac and that your Mac meets the System Requirements for the version of OS X that you are planning on installing, which you will hopefully find listed on Apple’s Support web site.
Stage 1 – Download the Installer
You can download the installer at any time. You’re not required to start the installer once you’ve downloaded it. You could let the installer sit on your Mac for seconds or months. When you’re ready to start the download, please do this:
When the App Store opens click the Featured button on the left end of the top row.
Typically there is a big ad for the current version of the Mac operating system (macOS) such as OS X Capitan or macOS Mojave. If prompted, enter your AppleID password. Sit and wait for the download to complete. The speed of your Internet connection will affect how long it takes. When the download finishes an app named “Install macOS” will open. You can either start the install immediately or quit it and do Stage 2 in the future.
Stage 2 – Run the Installer
When you’re ready do this:
Go to your Applications folder and find the Install OS X El Capitan application.
Double click it and follow the prompts to let it run.
You’ll be asked to enter your Mac user account password early on.
Once the install starts you won’t be able to do any other work on the Mac. Just let it run. It’ll take about 45 minutes or so to finish. It’ll restart your Mac when it’s done.
After your Mac restarts, login to your user account. Depending upon which features you already use, you’ll likely be prompted with a couple of on-screen questions. As a precaution, I recommend that you do not enable any of these features that you are asked about.