How to switch negative image to positive image in iOS 9 on my iPhone 6?

hi
My little 3 year old boy probably changed some features in setting. At the result, my iPhone screen image is negative. Can anyone know how to switch negative image to positive image? I am using iOS 9.
tHANKS

You need to turn the Invert Coluors Off (Settings > General > Accessibility > Invert Coluors)