how should I downgrade blackberry o.s from 10.3.1 to 10.2 in z3. 10. 2 have better battery.

Please help send me direct link form where I can downgrade from 10.3.1 to 10.2

Aside from the fact that I'm not sure you can even go back that far, why are you wanting to downgrade?

You can't go back once updated. Especially that far.
There are many forum threads to help you with your battery woes.

BlackBerry Classic, Passport and Z30 :).