Can I update IOS on a iMac early 2009
CAN I UPDATE IOS ON iMac early 2009
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The last version of Mac OS an early 2009 iMac will run is Mac OS (note NOT IOS which is for IOS devices) is 10.11.x you can use How to get old versions of macOS - Apple Support if you are not up to 10.11.x. Your computer is simply too old and does not have the hardware (sorry it cannot be upgraded) to support later versions of Mac OS.
Have you considered a newer Mac?