What I mean by quickly is this: Apple "signs" versions of iOS. This tells the device that the version you wish to use is okay to use with that device. While that version is signed, you can install it on your iOS device — even if it's a version of the operating system that's earlier than the one you just installed.
However — and this is very important — Apple traditionally stops signing old versions of iOS just a day or two after releasing major updates (such as iOS 8). If you were to update to iOS 8 say, a week after it ships, you might find that you can't go back to iOS 7. So, if you intend to be among the earliest adopters, it's not a bad idea to have a copy of the iOS 7.1.2 .ipsw file at hand.
If you have a copy on your hard drive you will find it, by default, by following this path: youruserfolder/Library/iTunes/iPad Software Updates. (You access the Library folder in your user folder by holding down the Option key in the Finder and choosing Go > Library.)
The Mac will delete this files under certain circumstances so you may not have a copy of the latest one. If not, launch your favorite web browser and search for "download ipsw". You'll find a number of sites that offer links to the file you need. Just be sure to choose the correct one based on the device you're using.
When we discuss upgrading to iOS 8 in that future article I'll also provide steps for rolling your device back to iOS 7 (at least for as long as you're allowed).
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