How to stop a system update upon restart

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 that a restart will not finish installing the update 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?