how to keep an object at a certain speed in unity code example

Example: unity get speed of object

public float speed;
public float speedPerSecond;
public Vector3 oldPosition;

void Update()
 {
     speed = Vector3.Distance(oldPosition, transform.position); //This is the speed per frames
  	 speedPerSecond = Vector3.Distance(oldPosition, transform.position) / Time.deltaTime; //This is the speed per second
     oldPosition = transform.position;
     Debug.Log("Speed: " + speed.ToString("F2"));
 }