I just bought a i9250 Nexus, and the system proposed to upgrade to newer version 4.2.1 (it is not a rooted phone. It is an unblocked UK phone)

I played with the new phone and after a couple of days I did upgrade it.

Now the camera is not working. When I try to use the camera, it calls the application and before I can see any image from camera, it says "Unfortunatelly the GALLERY has stopped"

How can I downgrade to previous version? (The phone is not rooted)

Or, does anybody knows how to fix it? or anything that I can try to bring the camera back to action?