Keep your Mac up to date
Periodically, Apple releases updates to your macOS software, which can include updates to apps that come with your Mac and important security updates.
If you receive a notification that software updates are available, you can choose when to install the updates or choose to be reminded the next day. You can also check for macOS updates manually in the Software Update pane of General 
settings.
Tip: To check for updates to software you’ve downloaded from the App Store, open the App Store.
Check for updates manually
To install updates manually on your Mac, do one of the following:
To download macOS software updates, choose Apple menu
in the sidebar (you may need to scroll down), then click Software Update on the right.To update software downloaded from the App Store, click the Apple menu — the number of available updates, if any, is shown next to App Store. Choose App Store to continue in the App Store
app.
Set your Mac to check for software updates automatically
On your Mac, choose Apple menu
Click Software Update on the right.
To set update options, click the Info button
for Automatic updates, then do any of the following:To have your Mac check for updates automatically, turn on Check for updates.
To have your Mac download updates without asking, turn on Download new updates when available.
To have your Mac install macOS updates automatically, turn on Install macOS updates.
To have your Mac install application updates from the App Store automatically, turn on “Install app updates from the App Store”.
To have your Mac install system files and security updates automatically, turn on Install Security Responses and System files.
Click Done.
To receive the latest updates automatically, it’s recommended that you turn on Check for updates, Download new updates when available, and Install Security Responses and System files.
Note: MacBook, MacBook Pro and MacBook Air must have the power adapter plugged in to automatically download updates.