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
Open System Settings or System Preferences
Select General
Click Software Update
Wait for available updates to appear
Click Update Now or Upgrade Now
Enter your administrator password if prompted
Follow the on-screen instructions
Allow the Mac to restart if required
Keep the Mac plugged in until the update finishes
Check Software Update again after restarting
Update apps from the App Store if needed