Software Upgrade iMac, Mid-2009

You have one OS level remaining for a mid 2009 iMac: macOS 10.11 "El Capitan."

⚠️ Before doing anything, make sure your computer is running properly--no major glitches or persistent problems†--and BACK UP YOUR HARD DRIVE to an external device.

The download link for El Capitan is in the last section of this Apple support article:

How to get old versions of macOS - Apple Support

Use Safari for any downloads as other browser do not always work with macOS system upgrade files.

†— existing issues must be resolved before upgrading macOS. Too many people upgrade to escape issues and find themselves in the same hole, or sometimes one that is worse. If you are upgrading due to current issues, let us help you with that BEFORE you try upgrading.

Back up your data, check your applications for compatibility, and depending on which iMac model you have, click on El Capitan or High Sierra.

A 20-inch or 24-inch one can't be upgraded past El Capitan.

The actual mid-2009 iMac is an education-only model.

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A mid 2009 iMac is extremely obsolete however it can be upgraded to run 10.11.X which is also obsolete. You can get 10.11.x by clicking How to get old versions of macOS - Apple Support, please read this very carefully.

IMHO it would be very wise to consider replacing the computer with a more up-to-date Mac. You have many many options open to you which would dramatically improve performance and not break the bank. If you would like some suggestions please let us know and we can offer some that you many not have considered.