This is really one of the simplest things you will ever have to do(if I understand you correctly) If you want to do this without any scripting required, why don't you just change the variable from 5.0 to -5.0(a negative value)?:

using UnityEngine;
public class walkforward : MonoBehaviour{

public float WalkSpeed = -5.0f;

CharacterController controller;



void Start(){

    controller = (CharacterController) transform.GetComponent("CharacterController");

}



void Update(){



    Vector3 movVel = Vector3.zero;



    if (Input.GetKey("w")){

       animation.CrossFade("walk_forward");

       movVel = transform.forward * WalkSpeed;

    }

    else {

       //if we're not moving we'll want to crossfade back to the idle animation

       animation.CrossFade("idle");

       float verticalAxisInput = Input.GetAxis("Vertical");

       if (verticalAxisInput != 0)

         movVel = transform.forward * verticalAxisInput * WalkSpeed;

    }

    // move the character with specified movVel velocity

    controller.SimpleMove(movVel);

    }
}

There are many ways of doing it, and you don't know the basics(which is not a bad thing), but starting out with a few tutorials will work wonders and will properly help you with this problem.