I have a desktop macOS Sierra. How do I upgrade to later versions?

First, it depends on the exact model iMac you have. Check Apple > About this Mac and tell us what it says for iMac and processor.

macOS Sierra was released in 2016 so your iMac must be at least that old. You can probably upgrade to High Sierra 10.13 and you *may* be able to upgrade to Mojave, Catalina, Big Sur or Monterey, but not to Ventura.

You can upgrade to at least High Sierra 10.13. You need a 2012 or later Mac to upgrade beyond that.

See: How to download macOS - Apple Support

High Sierra is the latest version you can run.

You're welcome.