Just got an iPhone 5s and it will not install the update at all

I just got my new iPhone 5a upgraded from the 5 had no trouble with the update on that device or my iPad. When I go to install it will download halfway n then say install n then stops where it says the update is not working. I plugged into iTunes n had the same problem but it had a longer load time like 4 or 5 hrs but then would a through an error message up bout the update not working.

Please help got this phone only awhile ago.