How to edit prefab instances at Runtime/Creation Time

Hello,

I had a query. How can i edit prefab instance at runtime/instantiation time. I can do instantiation at runtime but can't get any property to modify. I have a 2d UI image with button script also as prefab. I want to change the image and arguments for onClick event. I am using unity 4.6 beta i've just switched from cocos2d to unity. Actually i am trying to make a level menu in which dynamic level addition is required. Please help me on this.

Regards