It is important to know that jailbreaking your iOS device requires a version of iOS that can be jailbroken. Otherwise, the only chance to jailbreak your device is if you can downgrade to an iOS version that can be jailbroken.
Apple doesn’t always make downgrading very convenient. Any iOS installation via iTunes requires a version of iOS that is signed by Apple. If Apple no longer signs an older iOS version, you will not be able to install it on your iPhone, iPad or iPod Touch, no matter what the current version running on the device is.
This is bad news for those who upgraded to iOS 10.2.1 as they are not able to downgrade to iOS 10.2. Since iOS 10.2 is the latest version that can be jailbroken with Yalu and Apple stopped signing it, you cannot jailbreak your device because you cannot downgrade to the older firmware.
Some of the jailbreak enthusiasts see a chance with Prometheus; it is a set of tools that enables downgrading to older iOS versions, regardless of them being signed by Apple or not. Even so, there are many requirements you have to meet in order to use it. Among other methods, Prometheus uses nonce collisions for downgrading, but they were thought to not be working on iOS 10.2.1. It turns out that this isn’t true, since nonce collisions do work on some devices, such as iPhone 5s running iOS 10.3 beta 1. If you are willing to try your luck, you need to do that on a device that produces nonce collisions, such as the iPhone 5s running iOS 10.3 beta 1 and of course, you must have iOS 10.2 .SHSH2 blobs saved while Apple was still signing the firmware.
After these requirements are met, you are ready to downgrade iOS 10.2.1 to iOS 10.2. The first step is moving iOS 10.2.1 to iOS 10.3 beta 1. After that, use “futurestore” from Prometheus set of tools to downgrade to iOS 10.2.
If you successfully downgraded to iOS 10.2, the instructions here will help you jailbreak it using Yalu102.