my iMac says I am using mac OS Monterey 12.0.1. How do I update to 14 or 15?

Monterey is the latest version of macOS for the Mac. iOS/iPadOS 14 &15 would be for iPhones and iPads.

You're welcome.

The simplest way to keep up to date with MacOS is to go to System Preferences and click on Software Update. If there are any updates it will allow you to do it.