The simplest way to do that, without code, is to create the object as a button symbol.
What you do is put the visual for the non-rolled over in the first frame (Up) on the button's inner timeline, and the second visual on the Over frame of the inner timeline. It's ready to work.
To get started, select Insert -> New Symbol and select a button as the symbol type. If you look at the timeline it will show four frames that you can use for the button's visual content, labelled Up, Over, Down, and Hit.
wow really easy thanks a lot
You're welcome. You may want to check back because someone else may offer another approach... I just offered the simplest I know of for what you described wanting.