If you have reached your 5-computer limit, then launch iTunes, click iTunes Store, and then click your iTunes user name in the top-right corner of the store window. Once you login, you should see a page with all of your account information on it. In the middle of the page, you should see how many computers are currrently authorized. When you reach your 5-computer limit, a new button will appear here called "Deauthorize All." When you click this button, and confirm, your computer limit will start over at zero.
You will then need to re-authorize the computers that you want to keep. From iTunes, go to Store > Authorize Computer.
Update: As many have noted in the comments, this deauthorize all button can only be used one time per year. So, if you only have one or two computers to deauthorize, it might be best to do it manually. You can learn more about iTunes de-authorization on this Apple support note.
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