Software Updates in devices
 

I want to know actually what happening when we upgrade a device software..for example we upgrade a device from Android ICS to Jellybean.We do download JellyBean from the server.Then we will install it.but my question is during installation does it replaces the ICS in memory or it resides in separete memory?.I know the Kernel is same for both ICS and JB.Can we downgrade to ICS from JB?

I know that is this is not exact place to ask this question.but i think it is a genric question..