How to reset icould security code?

Hi, I got a problem when i started my new ipad pro.

It required me to fill in the vertification code then my iphone received the code. But after i fill the code, it required me to fill the ICLOUD SECURITY CODE, and i tried several times then realized that i might forgot the code.(4 number)

How to reset the icloud security code? My iphone is ios 9.3.1, and i cannot find "Advanced" under the "keychain" option.

Thx for helping!