Apple has released iTunes 11.4 for those on Mac OS and Windows. The new version of Apple's multimedia suite adds support for iOS 8. This means that you should have iTunes 11.4 installed if you are planning to purchase an iPhone 6 or iPhone 6 Plus
You can download iTunes for Mac OS directly on your device by opening Software Update by clicking the Apple icon. If you own a Windows PC, you can run Apple Software Update from your Start Screen (menu) or by using the following link
Apple has already released iOS 8.0 GM to developers, which means iOS 8.0 has been finalized for release on September 17, as long as no major errors are discovered.
It is also important to note that Apple has released iTunes 12 beta with the latest version of OS X Yosemite. However since users are reportedly facing issues installing iOS 8.0 GM with iTunes 12 and Yosemite, it looks like iTunes 11.4 is the best way to get iOS 8 on your device for now.