Bad news for those looking to downgrade their devices to iOS 10.2.1 or iOS 10.3 as Apple has closed the signing window a while back. iOS 10.3.1 is the way to go now. 

Today's update of  iOS 10.3.2 Beta2 for iOS developers is available to beta testers through Apple's Developer Center as either a download or an over-the-air update through the developer profile. You can upgrade in 3uTools directly, here we go.

Step1: Download and install the latest 3uTools on your PC. Before upgrading, you should backup your iDevice. Click here to check the details. Connect your iPhone to PC with a cable. Then please click the button "Flash & Jailbreak”→ click the button "Easy flash". 

(Note: If you want to retain the data, please select the option " Retain User's Data While Flashing".)

Step2: Here, 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.