Step1: Download and install the latest 3uTools on PC. Launch 3uTools and connect your iPhone to it. Click Toolbox ->Backup/Restore -> Backup iDevice.
Step2: After iDevice is backup successfully, you need to flash your iDevice using 3uTools. Click Flash & JB -> select the firmware 10.2.1-> click Flash, and then just wait until this process is completed. Remember to activate your iDevice after flashing.
Step3: When iDevice is activated, please back to Toolbox, click Backup/Restore -> Backups Management -> Open location.
Step4: Find Info.plist file, and open it using notepad.
Step5: Find 14E5260b,and then replace it with 14C92.
Step6: Find 10.3, and then replace it with 10.2.
Step7: After modification, please back to Toolbox -> click Restore Data. Select the backups you just modified and restore it.