First try it with -> App Store… and then enter “macOS Ventura” in the search field. When macOS Ventura comes up, click on the title until you get to the window with the GET button. Click and it will start the download into your Applications folder.
As soon as the download finishes it will launch the installer (which can take a few minutes—be patient).
DO NOT proceed. Quit and save a copy to another location. Afterwards, you can reboot and the installation will resume, or simply double click the file in the Applications folder. In my experience, sometimes the download will happen from System Settings. I think they changed the interface from different versions of the beta.
Use the copy you saved to reinstall macOS whenever you see signs of degradation. In older versions of macOS, I would keep a copy of the newest combo updater and it usually works to perform the “refresh.”
If your setup won’t allow you to do this, you can go to
Download and install current or previous versions of the Mac operating system on compatible Mac computers.
support.apple.com
Available downloads go as far back as OS X Lion. Good luck.