Back up your Mac using 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 or System Preferences
Open General
Click Software Update
Wait for your Mac to check for updates
If an upgrade is available, click Upgrade Now or Update Now
Enter your administrator password if prompted
Follow the on-screen instructions
Agree to the terms and conditions if prompted
Wait for the download and installation to complete
Allow your Mac to restart if required
Sign in after the upgrade finishes
Check Software Update again to confirm the upgrade is complete