Revert Android Update 4.0 to 3.0/3.2
As I already wrote in my comment: A downgrade (as well as an upgrade) can be performed by flashing the corresponding ROM (for references, see other downgrade questions). To find a matching ROM, please see Where can I find stock or custom ROMs for my Android device?. For the process of flashing, you might want to check with the rom-flashing tag, especially the questions dealing with the SGS2.