iOS: We're fans of iOS 10, but not everyone's enjoying Apple's newest operating system. If you want to go back to iOS 9, you have a limited time window for doing so, so you should do it now while you still can.
Step One: Back Up Your Phone
In order to roll back to iOS 9.3.5, you need to wipe, then restore your phone. So you'll want to back up your device with iTunes or on iCloud first.
Step Two: Download iOS 9
You need to manually download the iOS 9 file for your device. Head to IPSW Downloads, find your device, then download the newest version of iOS 9 they offer. This will download an IPSW file, which is the iOS 9 operating system. When Apple stops allowing iOS 9, you will no longer be able to download it.
Step Three: Restore Your Device
Finally, plug your iOS device into your computer, open up iTunes, then click on your iOS device in iTunes.
Option-Click (Shift-Click on Windows) the "Restore iPhone" button in iTunes, then select the IPSW you downloaded in step two. Your iPhone will get wiped and restored as if its a new device.