how do I deauthorize one computer from itunes

For a computer running Mac OS X, move the cursor to the very top of the computer's screen, click on Store or Account in the menu bar, and choose Deauthorize this Computer.

For a computer running Windows, click on Store or Account in the menu bar(if needed, follow these instructions), and choose Deauthorize this Computer.

If the computer isn’t bootable, you can’t.

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If you’re able to log into it through Virtual Network Computing, you can use that. Otherwise, it’s not possible to individually deauthorize it.

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