I describe in detail how to open and install macOS Monterey. You go to Apple Menu > App Store, and then you search for macOS Monterey. If you have already downloaded macOS Monterey from the App Store, click open. Or, find the app Install macOS Monterey in the launchpad or the Applications folder in Finder. Either way, you double-click on the app, and you press continue. Then, you agree to the Terms of Use, select your disk, and enter your password. Next, it will start installing, and eventually, your computer will ask you to press restart.