How do I deauthorize all my computers in Itunes?

How do I deauthorize all my computers in Itunes? I have gotten a new computer every two or three years and then given them away when I got the next one. I just got a new Dell Precision Work Station and am trying to get ITunes up and going. However, I keep getting a message that I have already authorized the limit of 5 computers and that I need to deauthorize all of them and start over with my new one. I am struggling with the interface of the new ITunes and cannot figure out how to deauthorize the computers. Please help.

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