I need to de register my kindle because I lost it in traveling.

Just go to the account You need to delete your existing account Every thing will get done. Apart from this if you buy a new kindle or face some issue while de registering your account you can ask for the support
Kindle Technical Support Registration 877 690 9305

This is from M. Francis Professor of Kindology on Amazons customer service forum

Standard advice for lost/stolen Kindles:

You want to deregister your device first and foremost to prevent someone from purchasing items with your account.

Immediately go to your 'Manage Your Kindle' page:
http://amazon.com/manageyourkindle

Scroll down the left-side menu and select 'Manage Your Devices'
Find the Kindle that is lost/stolen.
**BE SURE TO WRITE DOWN THE SERIAL NUMBER OF THE DEVICE, FOR FUTURE REFERENCE**
Then click 'Deregister'

** You will need that serial number when you contact Customer Support, and to identify your device from a Lost & Found, and also it will be needed if you file a police report or file an insurance claim to your homeowners or rental insurance policy.

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Then, contact Customer Service and report the loss/theft. Amazon cannot help you locate your Kindle, but they will use the serial number to block the device from being registered in the future by someone else. Do this soon, because if the person who might now have your Kindle already registered it to their own Amazon account, it cannot be blocked from registering at your request any longer.

If you find your Kindle later, contact Customer Service again to reverse the process. To contact CS, click the orange 'Contact Us' button on the right side of this page, or use this link:
https://smile.amazon.com/gp/help/contact-us/kindle-help.html

Don't use the email option, but use the Phone option (if available from your location) or Chat option. By using the Phone option, after you send your phone number in, an Amazon Representative will call you back in a minute or two, sometimes sooner.

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If you have Active Subscriptions:
You will also want to make sure that you cancel any active subscriptions. Canceling the subscription will ensure that you are no longer billed; you'll also receive a pro-rated refund for remaining issues you already paid for. You always have the option of subscribing at a later date.

Go to your 'Manage Your Kindle' page:
http://amazon.com/manageyourkindle

Then, scroll down the left-side menu and select 'Subscription Settings'
For each subscription you have, use the 'Actions' button to the right of the title and select 'Cancel Subscription'.