Question

how we can change the velocity of an object

Answer 1

Velocity of an object is the rate of change in displacement of the object in time.

Since the velocity is a vector quantity, it has a magnitude and a direction too.

An unequal change in the magnitude of displacement in equal time periods ,

or a change in the direction ,

or a change in both will change the velocity of an object.

Answer 2

A change in speed, or a change in direction, or a change in both speedand direction means that the objecthas a change in velocity. Understand that in physics this means if you turn a corner, even if your speed is constant, your velocity changes. Often thevelocity of an object is not constant.