Kindle Fire HDX & HD Swipe down the bar at the top of the screen and choose “Settings“. Select “Language & Keyboard“. Select “Keyboard Settings“. Set “Auto capitalization” and “Auto-correction” to “On” or “Off” as desired.
I always disable auto-correct, spell check, grammar check, predictive text and any other error prone crutches from my devices. Totally not needed for those of us who actually passed Third Grade English.