Need Help installing windows 8.1

Hey Hope u guys can help me

I have a hp laptop that previously had windows 8.1 and magically upgraded to windows 10. I want to go back to 8.1.

It has since passed the 30 days to just do the one button roll back to windows 8.1.

I have no recovery image saved of 8.1.

If I copy my windows key that came with my laptop and download a iso of windows 8.1, how do I install 8.1 on 10?

Thanks hope someone can help I'm dying this Os gives me ulcers

Please delete if this is a bad question