Changing Movie Clips on Keystrokes

Hello,
Does anyone know how to change a movie clip to another movie clip, once that a directional button is clicked. In other words how to change a movie clip from a previous one that is being controlled. I know it has something to do with the IF function, but how can you change the current Movie Clip to a specified one. This would be really helpful thanks!

[center][size=1]He lives in a distant mind…[/size][/center]