for now.
10.7 is a paid for download that can be had in the App store.
You need to upgrade from 10.5 to 10.6 before you can upgrade to 10.7 (Leopard > Snow Leopard > Lion)
P
Just purchased the iPhone 4S. Using a Mac Book Pro with OS X 10.5.8. What do I need to use iCloud on the iPhone and iPod touch? I downloaded OS X 10.7.2. Before I could install it, it said I needed OS X 10.7.1, which I downloaded. Before I could install THAT, it said I needed to get OS X 10.7, but I can't find it anywhere. Can you please tell me where I can find version 10.7, or if I even need it.

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