Downgrade iOS 9.3.1 to iOS 9.2.1. You can't do it.
Apple closed iOS 9.2.1. If you have iPhone, iPod touch and iPad running on iOS 9.3.1 or iOS 9.3 - you can't downgrade iOS 9.3.1 to iOS 9.2.1. Now is not possible to downgrade the operating system version with a fresh iOS 9.3.1
We know that Pangu team don't have tools for Jailbreak 9.3. Currently, Apple is testing the update iOS 9.3.1 and new update iOS 9.3.2 beta, which makes changes to the system, some mobile applications ("Health», Apple News, Apple Car), and adds the Night Shift option, designed to improve sleep iPhone and iPad owners. Focusing on the GPS data and the clock, the software will calculate when it's getting dark, and change the color on the screen to facilitate sleep.
Don't forget that iOS 9.3 has big bug - bypass iOS 9.3. It's mean that very strange situation - iOS 9.3 has bug, but you can't downgrade from 9.3.
Ok. If you have iOS 9.2.1 - you can use all apps on your devices, but if you have iOS 9.3.1 or iOS 9.3 - you can use your devices with such version iOS.