You Can Downgrade to iOS 9.0.1
The iPhone 6s arrived with iOS 9.0 on board which means there is no way to drop down to the iOS 8 update. You can't drop down to the iOS 9.0 update either. Apple stopped signing off on that update last week after the iOS 9.0.2 release.
You can drop down to the iOS 9.0.1 update if you want. Apple is still signing off on the update which means the downgrade loophole is still open. Apple typically closes up the loophole after one to two weeks which means it could happen any time. You'll want to keep an eye out if you're interested in dropping down.