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Good News is, your Mac can handle the latest 10.12 Sierra.
Follow the steps below to upgrade the OS X.
Click on the Apple icon and App Store. This will bring App Store. Click Updates tab. You will see macOS Sierra there with FREE UPGRADE button. Click Free Upgrade and enter your Apple ID and password.
This will download the update. It may take a few hours to download. Once the download is completed, it will bring the OS X Installer. Follow the onscreen instructions to install it. It may take up to 3 hours to install the upgrade.
If you have any questions or would like any additional information, please don't hesitate to ask.
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