This article explains how to change the keyboard on a Fire tablet. The instructions apply to all Amazon Fire tablet models.
$50 Amazon Fire Tablet – How to Switch to Google Keyboard
To replace the stock Amazon keyboard on a Fire tablet, follow these steps:
Go to the Amazon Appstore on your device and download a third-party keyboard app. A search for “keyboard” will yield dozens of free apps.
Some keyboard apps install automatically after you download them, while others require you to open the app to install them. You may want to grab a few so you can try out different ones.