Update to iOS 6.1.2 not happening on brand new factory unlocked iPhone 5

I have a brand new factory unlocked iPhone 5 which was recently purchased. I am unable to upgrade iOS 6.1.2 on this new phone. The current iOS on this phone is 6.0.2. I have placed a local carrier SIM and it is working. Currently phone is in working condition with iOS 6.0.2 and I am trying to upgrade to iOS 6.1.2 which is the latest one. Kindly share your thoughts or experiences in upgrading to iOS 6.1.2 success.