Downgrade from iOS7(Final) to iOS 6.1.3?
You can downgrade by following the steps here (it's for iOS 7 beta, but will work for the release version too).

Keep in mind that Apple usually stops signing the older version of iOS shortly after the release of a new version. Until the time Apple continues signing iOS 6, you will be able to downgrade the OS. Nobody can predict when Apple would stop signing iOS 6.

Once Apple stops signing iOS 6, it wouldn't be possible to downgrade (unless you look or wait for jailbreaks, hacks and such solutions, which would depend on the iPhone model you have).