How to update older macOS to new apple MacOS features and apps
How do I update my very outdated MacOS (2009)computer to preform and allow me to use new apple AppStore apps and features???
Earlier Mac models
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You don't indicate which computer model you have. But a 2009 iMac, for instance, can only be updated to 10.13.6, which is High Sierra, and is quite dated. Many App Store apps require much newer versions of MacOS, and some web sites won't work properly. A MacBook Air from 2009 can only go to 10.11.6, El Capitan, even older and more limited in what it can do. None of these can run the latest Adobe, Office, or tax softwares.
The best you can do is update to the latest version of the MacOS allowed, and use it as best you can, but these older computers are very limited in how they can be used. 2009 is 14 years old. My employer mandates replacing our desktop or laptop computers every three years. A new computer is of course the obvious solution. If that is not financially possible, investigate lower cost used and certified or refurbished computers but only from reputable suppliers like OWC or Apple.