EDIT - Note that it is not possible to force an app to use the iOS 6 interface on an iOS 7 device using Xcode 5 (unless you make custom graphics). It is also not a good design approach. Users on iOS 7 expect a certain UI style, that's why they updated; so give it to them, don't force them back into an older interface.
If you really, really need the pre-iOS-7 interface, you can download a copy of Xcode 4 and build with that. However, starting February 1, 2014 Apple will stop accepting apps built with anything earlier than Xcode 5.0 (I think I saw something about that in the iOS 7 dev docs, but can't find the reference again).
Keeping the iOS 6 UI in your app, but building for iOS 7 is fairly straightforward. Make sure that you've set your Deployment Target as iOS 6.1 or earlier. Your Base SDK can still be set as iOS 7 though. Now, just follow the instructions below for each of your interface files:
- Open your storyboard file or XIB file
- Open the Utilities Panel on the right side.
Click on The File Inspector Tab. You should now see something like this:

Next go to the Interface Build Document Section and change the Builds For
setting to iOS 6.0 and Later or whatever version you need.

Then change the View As
setting to iOS 6.1 or earlier:

Xcode will prompt you, just confirm that you want to convert to the older UI.
Make sure to run your project in the iOS 6 Simulator. Otherwise, the iOS 7 Simulator will override it's own iOS 7 UI style. To download the iOS 6 Simulator, go to the Xcode Menu Bar. Select Xcode, Preferences. Then click on the Downloads tab. Finally, click on the Simulator(s) you need to download:

Then Run your app on the iOS 6 simulator:

Unfortunately, even disabling the iOS 7 UI in Xcode does not override it on the device / simulator. Unless you design custom UI elements, there isn't a way to maintain your iOS 6 UI on iOS 7. But as I showed you, you can continue to edit it in iOS 6 and build for iOS 6.