mac os upgrade imac 2009 mac el capitan

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work. Safari will open a redirect and open the Apple Store where you can commence the download. If an error message appears, that is probably because one is using another Browser.

Make a Time Machine Backup Before attempting.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

If yours has a 20-inch or 24-inch monitor, it can't be upgraded past El Capitan. Otherwise, back up your data, check your applications for compatibility, and click on High Sierra.

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A late 2009 iMac will accept High Sierra. An Early 2009 iMac will not.