How to upgrade my iPhone 4s? - 1

I just ordered an iPhone 4s 8GB which comes with ios 5 or 6. If i try the upgrade option on the phone or iTunes it'll directly upgrade to iOS 8. I want to upgrade it to 7.1.2 but not iOS 8. How to do it? I saw from the website.apple is still signing the iOS 7.1.2 ipsw files for iPhone 4s.So i downloaded the iOS 7.1.2 ipsw file from apple website (around 1.21 GB) Is that even official and safe to restore that OS on my new iPhone 4s? And is there any other way?

Added (1). just ordered an iPhone 4s 8GB which comes with ios 5 or 6. If i try the upgrade option on the phone or iTunes it'll directly upgrade to iOS 8. I want to upgrade it to 7.1.2 but not iOS 8. How to do it? I saw from the website.apple is still signing the iOS 7.1.2 ipsw files for iPhone 4s.So i downloaded the iOS 7.1.2 ipsw file from apple website (around 1.21 GB)… Is that even official and safe to restore through iTunes that OS on my new iPhone 4s? And is there any other way?

If iOS 8 comes, you have no way to upgrade to a lower iOS like iOS 7.1.2 you mentioned. Trust me, I got a terrible time in this problem.