The update is available via Software Update on your device or via the iOS Dev Center. It weighs in at 359MB for the iPhone 5s.
WHAT'S NEW IN IOS 8:
There's a lot of changes in iOS 8. Here's a few of the changes we've posted on already: Health, iCloud Drive,
Family Sharing, Photos, Camera, Mail, Safari, Messages, QuickType, Third-Party Keyboards, Interactive Notifications, and Contact Shortcuts.
We'll be taking a look at even more new features shortly. You can follow iClarified on Twitter, Facebook, Google+, or RSS for more.