Hi my name is ***** ***** to Just Answer. I am reviewing your question and will reply ASAP.
Downgrading is not recommended, and is not supported by Apple officially.
I am checking if there are alternative ways, thanks for your patience.
Unless you have a working backup in iTunes from when iOS 9 was installed you won't be able to downgrade to iOS 9.
If you do you can follow the steps "Go back to the current version of iOS" from this Apple article to go back to iOS 9.
If you have any specific issues with iOS 10 you should really try to resolve those instead of going back to iOS 9. You can ask me anything about iOS 10 that is not working how you like it.
If you decide you want to to go back to iOS 9 then try the steps in the article I gave you above, and if you have any questions please get back to me.
Thank you. David.