I'm fairly sure this is not possible but had to ask anyway.
My wife's iPhone SE was on iOS 9 which she loved, and she hates iOS 10 (has tried it on my phone and can't stand it). Today her phone popped up a dialog to upgrade and her finger slipped just enough to hit "OK" rather than "Cancel" (or whatever the precise wording, I forget). Now she's on iOS 10 and is absolutely miserable.
Is there any possible way to revert? I'm aware of the new "prometheus" option but as she's never jailbroken or had any interest in such things, she has no saved blobs for her phone from any prior OS version. I know this is a long shot but... fingers crossed.