How to install or upgrade iOS 8

Here is the Cnet.com  described on own article, How to easy to install or update ios 8 in you iphone, ipad.

Method 1: Over-the-air update

If you are on a Wi-Fi network, you can upgrade to iOS 8 right from your device itself. No need for a computer or iTunes. Go to Settings > General > Software Update and click the Download and Install button for iOS 8.

Screenshot by Matt Elliott/CNET

After tapping the Download and Install button, you'll need to tap to agree to Apple's terms and conditions and then wait as your phone slowly downloads the file.
Once you have successfully downloaded the file, which was 1.1GB for my 32GB iPhone 5S (though my iPhone told me I needed to have a whopping 5.8GB of free space in order to proceed), you'll be greeted by an Install Now button. After the update installs and your device restarts, you can slide to set up iOS 8. You'll need to click through the following setup screens:

Screenshot by Matt Elliott/CNET

Screenshot by Matt Elliott/CNET

Screenshot by Matt Elliott/CNET

Screenshot by Matt Elliott/CNET

Method 2: Via iTunes

If you are near your computer with iTunes, it might be faster to update this way. In my experience, updating my iPad Air via iTunes was faster than updating my iPhone 5S over the air. The first order of business is to update iTunes to version 11.4. Open iTunes and then click iTunes > Check for Updates to install the latest version.
Once iTunes is up-to-date, connect your iOS device. Click on your device in the upper-right corner of iTunes and then click the Update button.

Screenshot by Matt Elliott/CNET

A pop-up window will appear, alerting you that a new iOS version is available for your device. Click the Download and Update button. (Or, if you are going to need your phone in the near future, opt for the Download Only button to download it now and install later. iOS 8 is a 1.1GB download and took the better part of an hour to download and install via iTunes.)