In 2016 Apple was forced to pay $400 million dollars to customers who purchased e-books between April 1st, 2010 and May 21st, 2012. These funds were distributed to the leading online retailers such as Amazon, Barnes and Noble and Kobo. You need to check your Kindle Balance to see if you have any unused credits, because they expire on June 24th.
You do not need to do anything special in order to redeem your credit. It is automatically applied when you purchase an e-book from Amazon, but sadly the credit is not applicable to print. If you have fled to print from digital, you owe it to yourself to see if you can get a few e-books for free. Once the credits expire, they are gone forever.