This is a basic question about flash. Basically the coordenade system have origin (the point
(0,0)) at the top left corner of the
stage. So basically the vector representing the y is going down so increasing the y value makes the entity go downwards.
To make the enetity go upwards you simply have to decrease the value of y.
PD: Yes this coordinate system have the y inverted if you compare it to the most common system. But in math it's known that there are an infinite amount of systems and for screens this is a common one.