The 2010 iMac can be upgraded to macOS 10.13 High Sierra and no further.
❗️ Do NOT upgrade until you have made a Time Machine or other backup of your startup drive.
Learn How to Back up your Mac with Time Machine
How to Upgrade macOS:
The macOS version you can upgrade to depends on the year your Mac was released.
Click the (Apple) menu at the top of your screen and choose About This Mac.
Check the Compatibility of your Mac with newer versions of macOS:
➡️ macOS 10.13 High Sierra is compatible with the Macs listed here
✅ - IF your Mac and apps are compatible, then using Safari only, download the macOS upgrade installer app:
Click > High Sierra.
Once downloaded, double-click the “Install macOS…” app in the Applications folder to begin the upgrade.
Please see > How to download macOS.