You can de-authorize all computers by signing into your Apple account.
If you need to authorize your new computer and are unable due to already having five authorized computers, you can deauthorize all computers by doing the following:
Click iTunes Store on the left side of iTunes.
If you're not signed in to the store, click the Account button, then enter your account name and password.
Click the Account button again (your Apple ID appears on the button), enter your password, and then click View Account.
In the Account Information window, click Deauthorize All.
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