Downloading the newest version of Apple's software to your old iPad, iPhone or iPod isn't necessarily a great idea and here's why.
Apple has designed iOS 8 for the central processing unit in its new range of iPad and iPhone, not the older ones, which means it will run faster on the Apple devices or iDevices it comes preinstalled on. It also runs reasonably comparably on the previous generation of iDevices before it, the iPhone 5, 5S and 5c for example, but it will really gum up the works on older iDevices.
Last generation's iPhones 5S, for example, is around 10 times faster than the iPhone 4S and iOS 8, with its hundreds of new features, runs much more smoothly on the newer iPhone. On the iPhone 4S it's much slower, and is prone to freezing up while it tries to accomplish more processor heavy tasks.
iOS8 won't even run on the generations of iDevices before the fourth, neither would iOS7, meaning some devices are frozen in time and will never run any news versions of iOS.
That doesn't really matter since the device just carries on doing all those things it was originally designed to do in the same old way.
There's another reason why you might like to hold off on updating to iOS8, even if you are the owner of a fifth generation iDevice running the iOS7 that is fast enough for the new software. That is that no software is generally perfect or bug free and it takes software writers a little while after it has been released into the wild to find and get rid of the major bugs. They are not usually major bugs, but they can cause problems.
That's why iOS6 was such a short lives mobile operating system because Apple hadn't fully tested its Maps app which was full of misinformation. Now that was a large set of bugs.
With all that in mind you should think long and hard before deciding to update your iOS device.
iDevices generally update over wireless networks. iOS8 takes 1.1 GB of space on the device after installation You'll need around 5 GB free to download and install it. Download it by going to Settings>General>Software Update and you will receive a notification telling you whether iOS8 is, or is not, available for your device. It says, over wireless, it will notify you when the update had downloaded. It could take a while over wireless.
When connected to iTunes on your personal computer the update starts automatically and takes about eight minutes to download and a few more minutes to install. If you want the best of all worlds, the latest software on the latest hardware, the ultimate solution is to buy one of Apple's iPhone 6 or iPhone 6 Plus.
The big difference between them is the screen size and the iPhone 6 Plus has an image stabilisation function built into its camera.