Battery problem on iPhone 5

I have problem with battery life on my iPhone 5. Turned off on 35% to 40% for saving battery life but 5% to 15% low battery (red) when turned on. What s wrong with this battery life? Replace new battery? or reset all settings? I won't exchange new iPhone 5S I really want to buy new iPhone 6 which battery will be longer than iPhone 5.

Please help me. :(