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If your Kindle has version 5.1.1, then you are running the latest update. You can ignore the warning sent by Amazon. It is just a general notice and not specified to your device. You will be able to continue using it with Kindle services (*even after March 22).
You still will be able to download and read books, apps etc without any issues
You can find the software requirements here --> http://www.amazon.com/gp/help/customer/display.html?nodeId=201994710
The latest update is here --> http://www.amazon.com/gp/help/customer/display.html?nodeId=201596850
Since you are running 5.1.1, you don't have to do anything else.
If you have any questions or would like any additional information, please don't hesitate to ask.