Q: I have OS X 10.9.5, how can I update it to 10.10.5?

Depending on your Mac model, you can either click here and download El Capitan, or get Sierra from the Mac App Store.

If you specifically need Yosemite, either log into the Mac App Store with an Apple ID previously used to download it, or go to a physical Apple Store and ask for it.

You don’t need to go through an intermediate OS version to upgrade from 10.7 or newer.

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Apple no longer offer Yosemite in the app store, so its not available if i have already Purchased it...

Apple still keeps in-active products it in the "purchased" list for some reason, but u u try and download, u'll get an error saying its not available..

Its a "purchased" list granted, but just like with Apple music greyed out songs taken away because licenses expired or something, its confusing the user, since they still appear just any app does in the App store purchase history.,. I dunno why Apple does this. They need to really get their act together.

Apple still keeps in-active products it in the "purchased" list for some reason, but u u try and download, u'll get an error saying its not available..

Are you using the same Apple ID you used when you downloaded it?

Try resetting the App Store and Clearing Cookies.

Reset       Learned from Old Toad