Before I go ahead, I want to make one thing very clear, this is the first release of a jailbreak for iOS 10.2.1 and this is in BETA. This can bootloop your device, forcing you to restore to the latest version. Be careful about the tweaks you install. Moreover, this only supports a limited number of devices. IF you still want to go ahead, READ THE ENTIRE POST BEFORE YOU TRY ANYTHING
Supported Devices:
iPhone 6/6+
iPhone 6S/6S+
iPhone SE
iPhone 5S
With the new update (beta 3), Saigon works much better on iPhone 6S/+ and iPhone SE, and supports beta 3. If you have an iPad, stick to beta 2
Now, if your device is a part of these devices, let us proceed with the "how to" of the part.
Instructions:
1) Download the IPA from the site : https://iabem97.github.io/saigon_website/
2) Download Cydia Impactor from http://www.cydiaimpactor.com/. iTunes should be installed on your PC/Mac for this.
3) Open Impactor, connect the device with USB to your computer. Drag Saigon.ipa to Impactor and put your login details.
Note
It is suggested to use a burner email for this.
4) In Settings -> General -> Profiles and Device Management, trust the new certificate.
5) After it shows complete, in your iDevice, open it and press the Jailbreak button. If it shows an error, reboot and retry.
Additional Note
If your device is an iPhone 6S or SE (and maybe Air 2), then it may reboot at Privilege Escalation. Go in to Airplane mode, hard reset, then toggle Airplane mode fast, then open Saigon and retry. If you get a lot of errors while trying to escape sandbox, disable Low Power Mode and connect to a charger (any charger long as it charges).
This might take you a lot of time to succeed. I personally had to try 50+ times to jailbreak once. Rejailbreaking is not easier either
Credits :
-xerub
-Adam Donenfeld
-Ian Beer
-Luca Todesco,
-Luca Todesco,
-Siguza,
-Tim Michaud
-Abraham Masri @cheesecakeufo