All instances in GameMaker: Studio have certain "built
in" properties that you can use and set to govern how they look and
behave. vspeed is one of those properties and defines the
vertical movement speed of the instance in pixels per step. So, a
vspeed of 3 means 3 pixels of movement down the screen
(+y) every step, and a vspeed of -3 would mean 3 pixels of
movement up the screen (-y) every step.
if keyboard_check(vk_up) vspeed = -5;
if keyboard_check(vk_down) vspeed = 5;
The above code will change the vertical speed depending on which keys are pressed.