How do I downgrade back to IOS 10 from IOS 11.2.1
how do i install an unsigned firmware (IOS 10) to my iphone 6S running on IOS 11.2.1
You can't. Only signed firmwares can be done on this phone.
While update firmware the modem firmware also upgraded so, downgrade is not possible on.
Apple has already turned off servers that digitally signed iOS 10 to allow for installation. Without this signature it is impossible to install iOS 10. If you used tiny umbrella to save those already signed signatures, you would still be able to downgrade to iOS 9 because the way the iOS signing occurred differently.