If an object is moving at a constant speed in one direction, what is needed to change its speed or direction?

Newton's 2nd law basically says that an object accelerates when a net forces act on it and velocity is speed in a particular direction. If a body accelerates, there is a change in velocity which means that either the speed or direction or speed and direction of the object changes.

A force is required to change speed and direction (Newton's First Law of Motion.)