To understand how velocity changes, we need to know:

* What are "a" and "b"? Are they locations? Points in time? Something else?

* What kind of object is it? Is it a ball thrown in the air? A car driving on a road? A rocket in space?

* What forces are acting on the object? Gravity? Friction? Air resistance?

Once you give me more information, I can explain how the object's velocity changes as it travels from "a" to "b".