iOS 9 on track to become Apple's most popular software ever

iOS 9 has been out for less than a week, and even though some reported problems installing the upgrade right out of the gate, Apple says more than 50 percent of devices have already adopted the latest version of iOS.[ Related: 7 tips and tricks for iOS 9 ]That means iOS 9 is on track to become Apple’s most popular software upgrade of all time. Apple announced the adoption rate in a Monday press release about iPhone 6s and 6s Plus availability, which is also expected to break records. The p