how do I upgrade OX 10.7.5

Upgrade to macOS High Sierra - Official Apple Support — if you're upgrading Mac OS X Lion Server (the community that this question was posted in) to a more recent version of macOS, you'll also need to purchase a new version of the Server.app package.

Depending on what Mac Pro model you have, download El Capitan or Sierra from the Mac App Store. If neither of them are compatible with it, you can’t.

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