Although Apple made no mention of release dates for its upcoming operating systems during its Wonderlust event, it quietly added those dates to the associated Web pages for its core operating systems. macOS 14 Sonoma is scheduled for 26 September 2023, and everything else arrives today, 18 September 2023.
These links should get you started:
With the first batch of releases, I advise waiting briefly to ensure nothing unexpected hits the news. Then, upgrade when you have some time to explore the new features.
With macOS 14 Sonoma, I urge more caution, particularly for production Macs, until you’re confident the software you rely on is compatible. Regardless, make sure you have a backup before upgrading. For full details and help with upgrading from my friends at Take Control, read Take Control of Sonoma by Joe Kissell and Take Control of iOS 17 and iPadOS 17 by Josh Centers.