Back up your Mac with Time Machine or another backup method
Connect your Mac to power
Connect to a stable Wi-Fi network
Click the Apple menu
Select System Settings
Click General
Click Software Update
Wait for your Mac to check for updates
Click Update Now or Upgrade Now if available
Enter your administrator password if prompted
Agree to the terms and conditions if prompted
Keep your Mac connected to power and the internet during the update
Restart your Mac if prompted
Check Software Update again after the restart to confirm it is up to date