Have you tried using "3uTools?"
I had the same problem.
1.Download 3uTools
2.Install it(Obviously)
3.After Installation connect your IPhone in Recovery mode
4. In 3uTools select the option called "Flash & JB"
5.Select the option called "Easy Flash"
6. And after that you will see a list of available IOS versions for your IPhone.
7. Untick the option called "Retain User’s Data While Flashing" Beside Flash
8.Just tick the Circle of the version of IOS you prefer and click "Flash"
If not, try visiting your local apple store