HOW TO DOWNGRADE OR UPGRADE MAC OS

📌 How to validate OS version:

Choose Apple menu > About This Mac.
Look for the macOS version.

📌 How to upgrade macOS version:

From the Apple menu in the corner of your screen Choose System Settings.
Click General in the sidebar, and Software Update on the right. Or choose System Preferences.
Click Software Update.
If Software Update finds new software, click the Update Now or Upgrade Now button to install it.

5. Enter your administrator password, if asked. During installation, your Mac might restart, and you might see a progress bar or blank screen several times.

If Software Update says that your Mac is up to date, then no new software is currently available for your Mac model. Software Update shows only updates and upgrades that are compatible with your Mac.

Source: https://support.apple.com/en-ph/HT201541

📌 How to downgrade macOS version:

Make sure to fully backed up Mac first because the following procedure will erase the startup disk, applicable for laptops that needs data backup.

Shut down Mac laptop.
Power on the computer and immediately hold Shift + Option + Cmd + R. You’ll notice startup takes longer than normal as MacOS Recovery loads.
Once the macOS Utilities screen loads, choose Reinstall macOS (or Reinstall OS X) and click Continue.

4. Follow the prompts and choose your startup disk. Now click on Install.

5. Walk through the onscreen instructions to complete the rest of the installation process.

6. Once the installation is complete, restore your files from the backup you created.

Source: https://www.makeuseof.com/tag/downgrade-macos-version/