Steps:
1)Restart mobile using power+vol down key+menu key,you just see the downloading in yellow text.
2)Connect mobile to pc.
3)start odin Multi_Downloader.
4)Select Cooper v1.0.ops .
5) Click on one package ,then select package i.e http://www.hotfile.com/dl/148986856/f274ebd/S5830DDKQ8_S5830ODDKQ7_INU.zip.html
6)click on start .
7)Wait for competion it require 5-6 mins.
8)You are just with 2.3.6.