To do the actual installation, open the App Store program on your Mac. If you don’t see “OS X El Capitan” advertised right there on the home page, you can find it by searching for El Capitan. Click the Free Upgrade button, and then click again when it changes to say “Download” (see Figure A-1).
If you don’t have a fast Internet connection, or if your Internet provider imposes monthly data limits, you might not be thrilled about the prospect of waiting for a 6-gigabyte download.
Your best bet is to take your Mac to an Apple Store and use its free, fast Wi-Fi to download El Capitan. (Or go to a friend’s house, a library, a coffee shop…).
Figure A-1. Enter your Apple ID and password when you’re asked for it, and enjoy a mug of your favorite hot beverage as the download begins. If you have a typical broadband Internet connection, expect to wait about half an hour.
When the download begins, behind the scenes, the App Store puts a 6-gigabyte program called Install OS X El Capitan into your Applications folder and onto your Launchpad, if you have it.
At this moment, you can duck out of the installer to make a safety copy of the El Capitan installer, so you won’t have to download it again the next time. See the box on the next page.
You’ll know when the downloading is over, because the installer opens up automatically and presents you with the logo shown in Figure A-2 ...