Solution Steps

1. Check available storage: System Settings → General → Storage (need 15-20GB free).
2. Restart your Mac and try updating again.
3. Check internet connection is stable.
4. Try downloading update from App Store instead of Software Update.
5. Boot into Safe Mode: Hold Shift while starting → Try update in Safe Mode.
6. Reset NVRAM: Restart → Hold Option+Command+P+R until second chime.
7. Contact Apple Support for advanced troubleshooting.