It is available for iPhone (iPhone 4S and above), iPod Touch (5th Gen) and iPad (iPad 2 and above). To start downloading, head over to Settings > General > Software Updates. It takes about 30-40 minutes on 5Mbps Unifi plan to download.
UPDATE: Take note that iOS 8 would require up to 5.7GB of free space to update from the phone. Mashable has posted a solution that you can install with less free space by updating using iTunes on your computer. Just make sure you click on “Check for updates” and DON’T click on “Restore iPhone”.