I get the answer from the forum.
Locate your Kindle for PC in the "Manage your Devices" section and click the "Deregister" link to deregister. You'll be able to deregister devices that you no longer have access to here as well.
If you uninstall the Kindle for PC application, it will not change your registration. You'll need to use one of the deregistration methods above to disassociate Kindle for PC with your Amazon.com account.
But in Kindle 3, Kindle for PC still exist under the direction "Add other Device Collections" and I have install the kindle for iphone which never show up in my kindle 3.
What should I do to delete the kindle for pc from my kindle 3? (I have degenerated it from "manage my kinde")
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