how to change a row of itemrenderer dynamically?or a particular item of a row in flex

hi im newbie to flex..
im using itemrenderer in a list to use a Hbox..
n Hbox contains a image,label and button,...i v to change a particular row of a list...
To be more precise,i v to change the image of particular row of a list dynamically.
how ca i change dynamically the source of that image?
please anyone help me..