# Interacting with box2D objects

How can I interact with an object in box2d, I'm completely new to this so I have no clue what to do. The box is created like this:

bodyDef.type = b2Body.b2_dynamicBody;
fixDef.shape = new b2PolygonShape;

fixDef.shape.SetAsBox(0.5, 0.5); // Half-Width, Half-Height

bodyDef.position.x =  5;
bodyDef.position.y =  5;
world.CreateBody(bodyDef).CreateFixture(fixDef);


How can I change the x, y positions of that box, tried bodyDef.position.x--; but it returned "bodyDef is not defined". How do I access this?