You Can Trial iOS 9
For a limited time, you can try out the iPhone 6 iOS 9 update. If you decide that you don't like it, you can drop back down to iOS 8.4.1.
This is accomplished using the downgrade loophole that always emerges immediately after an iOS release. If you decide that iOS 9 isn't right for you and your iPhone 6, you can use this loophole to drop back down to iOS 8.4.1.
Nothing older than that though.