my macbook air is 13-inch mid 2011. I could not find any update sofeware in my app store. How can I update? thx

If you mean upgrading macOS, High Sierra is the latest you can run.

How to upgrade to macOS High Sierra - Apple Support

If you mean some other app you need to be more specific.

Your computer can't be upgraded past High Sierra. If it's running an earlier OS, back up your data, check your applications for compatibility, and depending on which Mac model you have, and download it.

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