Apple already released iOS 10.2.1 beta4 for developers, and this beta version of iOS 10.2.1 contains bug fixes and improvements. This beta verison of iOS should only be deployed on devices dedicated for iOS 10.2.1 beta software development. 

Supported iDevices: iPhone 7, iPhone 7 Plus, iPhone 5, iPhone 5c, iPhone 5s, iPhone SE, iPhone6, iPhone 6 Plus, iPhone 6s , iPhone 6s Plus; iPad4, iPad Air, iPad Air 2, iPad Pro (9.7/12.9) iPad mini 2, iPad mini 3, iPad mini 4, iPod Touch 6.

Step1: Download and install the latest 3uTools on your PC. Connect your iPhone to PC using the USB cable. After it’s connected, please click the button”Flash Jailbreak”→click the button “Easy flash”. 

Step2: As shown in the following figure, please select the firmware you want, and then click the button “Flash ”. (If you have downloaded the firmware, you need to firstly import it to your PC, and then flash your iPhone).

Step3: Keep a good connection between iDevice and PC, and then just wait until this process is finished.