how to stop a system update upon restart Quick question,

I have an old Essential PH-1 that runs on Android 7.1.1 (Nougat). I accidentally okay'ed it to update to Oreo and it says to allow the updates to take effect to the newer Oreo OS I need to restart my phone. I do NOT want to update to the new OS. I want to keep Nougat OS.

How do I prevent this from happening?

Is there a way to erase these new files so when it eventually restarts it will not finish installing these updated files to Oreo? Or do I have to let it update upon restart and then downgrade  back to Nougat? If so, where do I go to find the easiest way to do that? Any help would be greatly appreciated.
You need to open your device developer option. In case it is not there then click your device build number 7 times. Then in developer option disable auto system update on restart.Link for the image