Thanks to MacInTouch for this information (with minor editing):
Please note that if you (and your friends/family/clients) have not downloaded OS X 10.11 El Capitan yet I strongly advise you to do so before Sept. 20 because after this date, in line with what has happened with previous OS X upgrades, it will probably no longer be available to download from the App Store. The download is free and you do not have to install it/upgrade to it:
1. Go to Apple menu > App Store… (or Macintosh HD > Applications > App Store).
2. Click the “Purchased” tab.
3. If you need to sign in, sign in with your Apple ID.
4. In your Purchased list look for “OS X El Capitan.”
5. If it is listed, you have already downloaded it and you do not need to do anything else – quit the App Store.
6. If it is not listed click https://geo.itunes.apple.com/gb/app/os-x…&at=11l7jb
7. If OS X El Capitan has a “Download” button click it to download (the installer is quite large ~6GB so may take some time to download).
8. Once it has downloaded the installer will launch.
9. At this point quit the installer – do not run through the installer.
10. In your Macintosh HD > Applications folder there will now be an “Install OS X El Capitan” application – you can leave that where it is unless you need the disk space in which case it should be safe to delete it (although it’s worthwhile to make a copy/archive of it somewhere else from your boot drive before deleting it).
11. The free purchase and download of OS X 10.11 will now be registered with your Apple ID so that if Apple remove OS X 10.11 next week from the main App Store you will have it in your Purchase history and will still be able to download it in the future should you ever need it.