Iphone 6 plus keyboard letters uppercase
Apple changed the keyboard in ios 9 to make it easier to tell when the shift key is pressed. In ios 8 and below, the keyboard letters are always uppercase, but in ios 9, they are only uppercase if the shift key is pressed.

You can update to ios 9.1 by opening settings, then choosing general > software update, and choosing "install". It will take a while, so you should probably plug your device in first. This should make the keyboard match your other device.