Hi! why can't i update from yosemite to a newer version

The MacBook Air mid 2011 can't be upgraded to the current macOS Catalina but it does support High Sierra 10.13.6. See the following instructions: How to upgrade to macOS High Sierra - Apple Support

Back up your data, check your applications for compatibility, and download High Sierra through the link provided. If you encounter a problem, it depends on what that is.

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