Help with 2nd bootloader (noob here)

Ive searched and now im 100% certain i have a Fire HD 8.9 inch 2nd gen tablet, It has update 8.5.1 on it, Im trying to figure out if i need to somehow downgrade or if i can just install 2nd bootloader on this software version. Sorry if this is already answered i looked and didnt see it, but i did a good bit of research before asking this question, but i just wanna be 100% sure of what i need to do.

Any help is greatly appreciated!

Thanks,
deershoot3r