How can I change vector speed?

Sorry for my English Firstly this is a cube script and it is Instantiate randomly location. cube must go randomly.if it is collide another cube or wall ,it must change its speed and direction. I did this script and it can go randomly direction but it doesnot change speed and sometimes it stop when it collide the wall.I work 2 days on this 2 problems.Please help me ! NOT:I use 5.3.1p1. Wall and cube has box collider.Thank you for your answer. using UnityEngine; using System.Collections; using UnityEngine.UI;

public class CanScript : MonoBehaviour {

 public Rigidbody rb;
 public Vector3 git;
 public float speed=5f;

 void Awake()
 {
     rb = GetComponent<Rigidbody>();
 }

 void Start()
 {
     git.x = Random.Range(-40, 40);
     git.y = 0f;
     git.z = Random.Range(-40, 40);
 }

  void FixedUpdate()
  {
      rb.velocity = new Vector3(git.x, 0, git.z);  // i cant change speed.i want to use it
  }

 void OnCollisionEnter (Collision col)     // 
 {
     if (col.gameObject.tag == "objetag")
     {
         speed = Random.Range(5, 40);   // i want to use this speed
         git.x = Random.Range(-40, 40);
         git.y = 0f;
         git.z = Random.Range(-40, 40);
     }
 }

}