iPhone 6 and iPhone 6 Plus are much larger to be used comfortably to reach all corners of the screen one-handed. To solve this problem, Apple introduced a feature called “Reachability” which is a software hack that let you move the user interface half-way down the screen when you double-tap (not double click) on the home button.
Reachability is time-saviour especially when you’re trying to tap on the left-top or right-top corner button using one hand. This feature is enabled by default and is available only on iPhone 6 and 6 Plus. If you’re one of those few who can’t ever get used to it, you can disable it.
Goto Settings app > General > Accessibility. Scroll down and look for the Reachability switch. Turn off the switch to disable this feature.