my iMac macOS is High Sierra. Can I upgrade to another OS?

Back up your data, check your applications for compatibility, and if your iMac is a 2012 or newer model, download Mojave, Catalina, or Big Sur. If not, no.

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High Sierra 10.13 is the latest macOS that a Late 2009 iMac can run. If you need to upgrade to a newer macOS, then you will need a new or newer Mac.