How to find Library Control in Action script

programming flash
1

i am beginner to action script is there any one who can teach me how to find library control in action script.

I want to change instance of symbol dynamically. like if the symbol1 is the instance of banner1.jpg now i want to change the instance using action script to banner2.jpg so now image of banner2 will display at runtime in symbol1 object

Thanks in Advance