For a rocket on the ground to change its velocity in an upward direction, what must it do?

According to Newton's first law of motion, an object will continue in its state of motion until and unless an external force is applied to it. This means that an object at rest will remain at rest, while an object in motion will stay in motion until and unless an external force is applied to it.

Similarly, a rocket that is on the ground is at rest (zero speed). To change its velocity in an upward direction, that is to push it towards the sky, we need to provide it with some force. Earth's gravity constant pulls on everything on Earth and this causes objects to fall down when we let go of them. Similarly, the rocket is also experiencing a pull towards the Earth. To move it in the upward direction, we need to generate enough force (F = m x a) to overcome Earth's attractive force; only then can the rocket accelerate. 

Another way to think about the problem is this: the rocket initially has zero velocity and ultimately has some velocity. This change in velocity over a finite duration is known as acceleration.

Thus, for a rocket on the ground to change its velocity in an upward direction, it must accelerate.

Hope this helps.