Mac Support

No such thing as macOS 15.

iOS 15 as shown in the support document you came form is for iPhones, iPods and iPads, not Macs.

The current macOS version for Mac is macOS 12.1 Monterey. If you have a compatible Mac, you can follow the steps here to install it: Link-> How to upgrade to macOS Monterey - Apple Support

There is no macOS 15 -- the latest and current version is macOS Monterey 12.1

You can check the following to see if your iMac is compatible and for instructions: How to upgrade to macOS Monterey - Apple Support

If you meant macOS 10.15 Catalina, that can be installed on late 2013 and earlier iMacs. The model year of your iMac is at Apple menu > About this Mac. Also see: How to get old versions of macOS - Apple Support

As both pervious posing have outlined.

Though if one really meant to upgrade and iDevice like iPhone for iPad Update your iPhone, iPad, or iPod touch