1) Open System Settings.

2) Click General. Then, choose Software Update.

3) To the right of "Automatic Updates", click the information icon (the "i" with a circle around it).

4) Next to each item listed, slide the switch to the "on" position (that is, to the right). Then, click Done.

5) If prompted for authentication, type the username and password of an account on your Mac with administrative rights. Then, click Unlock.

Outcome

The operating system on your Mac, as well as your apps from the Mac App Store, are automatically kept up to date.