When you update via Wi-Fi, all of your settings, media, data and apps are left intact, but the update may take some time; potentially several hours depending on the size of the download. If time is of the essence, there's another way. Plug the device into a PC or Mac running iTunes and use the option to either Restore or Upgrade. With the Upgrade option, your settings, data, media and apps are left in place. With the Restore option, everything on the device is deleted before a fresh version of the operating system is installed. If you've been having issues with your device, or if you've modified the OS in ways Apple hasn't sanctioned, a Restore may be your only bet.
Once the upgrade is complete and your device rebooted, Apple's Setup Assistant will guide you through the process of connecting to a Wi-Fi network, enabling Location Services and tweaking some other options. If you chose to perform a Restore, there are options to set your device up as new or to restore from a backup stored on iCloud or iTunes.
After a few swipes, a couple of password entries and a reboot or two, iOS 9 leaves you at the Home Screen, which looks pretty much as it did before. As with iOS 8, the interface theme is still built around a bright and colorful palette; navigating the operating system is still designed around the concept of layers, where tapping apps and folders zooms in and out of virtual 3D planes; the frosted white interface elements are still influenced by the colors of the current wallpaper; and the parallax effect (in which app and folder icons slightly shift depending on how the device is held) remains.
As with OS X 10.11 "El Capitan" on the Mac, there are a few new features, such as the revamped Search screens, which can be accessed from the Home Screen by swiping either down for a quick search or left to right for a more detailed view. But iOS 9's most important aspects revolve around refining the technology that was already present.
Better performance, more storage
The built-in iOS apps have all been rewritten to take advantage of Metal, Apple's API that was designed to utilize graphics more efficiently and effectively. Metal takes the best bits of OpenGL and OpenCL and combines them to provide low-overhead access to the GPU for both graphics and data processing.
The result is that iOS 9 feels very fluid and responsive: Animations for zooming in and out of apps and folders, scrolling through lists and using the multitasking view to switch between apps are all smooth and devoid of any stutter. Interestingly, the only system app I encountered that still dropped frames while scrolling is the new News app (which replaces the Newstand app). Also, third-party apps will need to be updated to take advantage of Metal.
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