Upgrading El Capitan

Hello TobiD,

Thanks for reaching out to the Apple Support Communities. You have an iMac either 21.5-inch or 27-inch from 2010. Both of the iMacs from 2010 can be upgraded to macOS High Sierra, which is macOS 10.13.6.

You may use step 4 in Apple Article - HT208969 to download macOS High Sierra from the App Store.

Your old Mac should be upgradeable to High Sierra: How to upgrade to macOS High Sierra.

Before You Upgrade macOS

1. Fix any problems before upgrading.
2. Run First Aid in Disk Utility while booted from your Recovery HD.
3. Backup. Make a bootable clone using CarbonCopy Cloner or use TimeMachine.
4. Be sure your startup disk has 18-20GB of free space during the installation.
5. Upgrade third-party software for compatibility. See App Compatibility Table - RoaringApps. If you have mission critical software, then DO NOT upgrade until you know that software is compatible with the version of macOS you are about to install.
6. Disconnect all third-party peripherals.
7. Download the installer which will be in the Applications folder. Quit the installer after it opens, then make a copy of the installer and save it in the Downloads folder. The one in the Applications folder will be deleted after a successful installation.
8. The download is large, anywhere between 5 and 9GBs. It takes time to complete. Be patient.
9. Double-click on the installer in the Applications folder. Click on the start button to begin the installation.

Back up your data, check your applications for compatibility, and download High Sierra.

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