iMac (21.5-inch, Mid 2010) How can we upgrade or download new Mac OS?
iMac (21.5-inch, Mid 2010) How can we upgrade or download new Mac OS?
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That iMac will run High Sierra, and no later. Newer versions are not supported on that iMac.
For newer versions past High Sierra, you’ll need a newer or new Mac.
If you’re not already running High Sierra, here is how to get it: How to get old versions of macOS - Apple Support
Have a backup created prior ro the upgrade, as that’s the only path to recover if there’s a problem with the upgrade, or if some critical app doesn’t work on High Sierra.