Please click on the link below for your operating system:
macOS Mojave (10.14) and newer
1) Click the Apple icon, then click System Preferences.
2) Click Software Updates.
3) Click Update Now.
4) Click Download & Restart.
5) Wait for the software update to download.
6) After the software finishes downloading, click Update Now.
7) Click Restart.
macOS High Sierra (10.13) and older
1) Open App Store.
2) If prompted for administrative credentials, type a name and password which have admin rights to the computer. Then, click Check.
3) Click on Updates.
4) Click the UPDATE button next to each update to install it, or click the Update All button to update all of them.
5) If presented with a license agreement, review the terms of the agreement. If you agree to the terms, click Agree.
6) You may be prompted to restart the computer. If so, save your work and close any open programs, and then click Restart.
Continue repeating the steps above until the Mac App Store software tells you that no updates are available.