NEED OLD IMAC SOFTWARE UPDATE

Check which iMac you have using About This Mac in the Apple () menu.

• If you have an Early or Mid 2009 iMac, you can upgrade as far as El Capitan, but no further.
• If you have a Late 2009 iMac, you can upgrade as far as High Sierra, but no further.

Then see:

How to download and install macOS - Apple Support

If your Mac can run High Sierra, and you run into problems with the App Store link (because your versions of Safari and the App Store are so old), you may need to upgrade in stages. I.e. by upgrading to El Capitan or Sierra (whose installers you can get in .DMG form), and then upgrading to High Sierra.

There were at least three separate and distinct iMac models released in 2009. To identify which one you have please read Identify your iMac model - Apple Support. Then, proceed with How to download and install macOS - Apple Support.

Back up your data, check the application compatibility table, click here, and then on the download link for a higher OS. Your computer's maximum one is El Capitan if it's an early or mid-2009 model and High Sierra for a late 2009 unit.

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