I downloaded the update but got a message that the installation failed:"Can't install the update".

That's the iPhone update, as the icon for the iPhone is shown.

Apple menu -> System Preferences -> Software Update can tell you if there are updates for your computer.

The last security update was released July 20, 2022. The September update for Safari on Big Sur and Monterey was not released for Catalina. Browsers by Mac operating system - Apple Community

OK. That was my wife's post. We have updated the iPhone. Hopefully that will solve the problem. Thank you.