How to update to macOS Catalina?

You can upgrade an iPhone 6s Plus to iOS 14.0.1 via Settings.

How to upgrade to macOS Catalina. From your Mac's menu : About This Mac panel, look at the product description line that also contains the Mac's product introduction date in parenthesis. Compare that date to the preceding hardware requirements link. If you Mac is too old, you cannot upgrade to Catalina.

Here you go: How to upgrade to macOS Catalina.