can i update my 2009 iMac

There was an Early 2009 iMac model and A Late 2009 iMac model.

see > Identify your iMac model - Apple Support

The Early 2009 20" and 24" models can run up to 10.11 El Capitan.

The Late 2009 21.5" and 27" models can run up to 10.13 High Sierra.

If you have a Late 2009, see > How to upgrade to macOS High Sierra - Apple Support

If your iMac is an early 2009 or mid-2009 model, it can't be upgraded past El Capitan.

If your iMac is a late 2009 model, it can't be upgraded past High Sierra.

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