Cydia iOS 9.3.1
First, let's talk about Cydia iOS 9.3.1. Cydia is a third party app store which developed by Saurik. It is not similar to the Apple app store. You have to jailbreak your device to Cydia download on your device. It is not so easy because in these days, Apple has patched jailbreak on their source code. However, you can follow steps to download Cydia on your device easily.
iNstant Jailbreak Official Website |
Apple patched untethered jailbreak iOS 9.3.1 version and there is no fully jailbreak for iOS 9.3.1 at this time. But, you can use iNstantJB tool to jailbreak your device and Cydia download easily with limited features and lots of applications to customize your iOS device.