To change speed you need to have an?

Speed is the ratio of distance traveled over a given duration of time. 

That is, speed = distance/time

To change the speed of an object, the object will need an acceleration (or deceleration) and this is only possible by application of force. According to Newton's First Law of motion, a body requires an external force to cause a change in its state of motion (from rest to motion or change in motion for an already moving body). According to Newton's Second Law of Motion, the force is a product of mass of the body and acceleration of the body. Hence, the speed can be changed only by application of an external force, which will generate an acceleration of the body. If the force is in the same direction as the motion, the acceleration will increase the object's speed. If the force is in the opposite direction (or has a component that resists the motion), the acceleration will resist the motion and the speed of the object will change.

Hope this helps.