What generation is it? Don't try any of this without confirming you have the right model and generation:
I've never been able to tolerate Amazon's bastardized take on Android. If you also hate it, You can make it a semi functional Android tablet by rooting, debloating and installing the Google Play services.
Here's how to root, use the "hardware mod" method:
https://xdaforums.com/t/unlock-root-twrp-unbrick-fire-hd-8-2018-karnak-amonet-3.3963496/
http://billm10.x10host.com/Fireroot.html
If you get really lucky, you might be able to flash LineageOS.
LineageOS for gen6-7:
And here's a tool to help with the question you asked (although I don't have any first hand experience, I prefer Google books):