create a new button. copy the graphics from the movieclip's frames to the corresponding button frames.
To convert a movie clip that is already a button:
1. in the Library right click on the movie clip
2. Select Properties
3. Click the radial button next to Button under Type.
4. if the mc is already on stage, highlight it and in the Properties panel change the Track As to Button
no need to copy frames. When you do the first 3 steps your timeline within the clip with change to a button's timeline.
you should ignore antonio's suggestion. i'm sure he's well-meaning but converting an object from one type to another, while usually ok, occassionally leads to problems that are impossible to debug without deleting the converted object and creating the needed class instance de novo.