I took my Kindle Fire out today and the screen was impossibly dark, making it hard to read the screen well enough to adjust settings.
On the homepage there is a settings button & inside the settings menu there is a link for display.
You can adjust display brightness there, and there is also a feature called adaptive brightness which you can turn on or off with a switch.
The catch with all the above advice is you first need to be able to see the screen well enough to access the on-screen buttons.
What I did was turn the flashlight feature on my phone on & shine it directly at the light sensor on the Kindle Fire. That made the screen bright enough that I was able to turn Adaptive Brightness off.
If you want to use Adaptive Brightness you still can, but to adjust it to be brighter you have to have it turned on and then click the same brightness adjusting slider that you click when it is turned off.