Update your macOS

1. From the Apple menu in the corner of your screen, choose System Settings. Click General in the sidebar of the window that opens, then click Software Update on the right.

2. In earlier versions of macOS, choose Apple menu > System Preferences, then click Software Update.
3. When opened, Software Update automatically begins checking for new software. If new software is available for your Mac, click the button to Update or Upgrade
4. If new software is not available for your Mac, Software Update says that your Mac is up to date. Software Update shows only software that is compatible with your Mac model. For example, if your Mac is not compatible with a later version of macOS, Software Update does not offer it to you.
5. Before installation begins, you are asked to enter your administrator password. It's the password you use to log in to your Mac. 
6. During installation, your Mac might restart and show a progress bar or blank screen several times.

Source: https://support.apple.com/en-us/108382#:~:text=From%20the%20Apple%20menu%20%EF%A3%BF,Preferences%2C%20then%20click%20Software%20Update.