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The latest version is macOS 10.12 Sierra. Good News is, your Mac can handle it without any problems. To upgrade to Sierra, follow the steps below;
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.
or click here to download Sierra directly --> https://itunes.apple.com/us/app/macos-sierra/id1127487414?mt=12&ls=1
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 difficulty on following the steps above, please let me know and I will continue to help you.
Note : This is not my final answer. I strive to provide Excellent service or 5 star service. If you feel you've received anything less or if you are not satisfied or if my solutions didn't work, please don't rate me bad or poor or ok. Instead, please let me know and I will try my best to help you until you are fully satisfied or the issue is resolved.