Hard reboot, and then try installing the firmware again.
If your smartphone has Jailbreak installed, update using iTunes.
If you get error 1671 when updating via iTunes, then the software is downloading to your PC, try updating a little later.
If the firmware was successfully installed, but the smartphone goes into a cyclic reboot, perform a restore via iTunes.
Click here to read in detail: https://repairbuy.in/blogs/error-while-updating-ios-what-to-do-and-how-to-fix-it/