One thing you'll need to learn eventually when you program games is kinematics. Generally, most objects in games have the following properties: position, velocity, and acceleration. Each time you call update you update the position based on velocity, and you update velocity based on acceleration. It would look something like this.
position += velocity
velocity += acceleration
If you only need your object to change direction instantaneously, then acceleration isn't needed. However, since you what your character to fall down gradually like objects do in real life, you will need acceleration. If your Y-component represents the up/down direction, you give your object's acceleration a negative value in it's Y component.
Your update code could look something like this:
position += velocity
veloctiy.y -= gravityConstant
