While some of the headline-grabbing features of iOS 7 will only be available on the new iPhone 5S, like the fingerprint ID function, there are many hidden tweaks and new features for older devices.

Apple (NasdaqGS: AAPL - news) 's new iOS 7 operating system has become the fastest software upgrade in history, according to the industry giant. More than 200 million devices are now running on the ...