How do you add a speed variable?

Hello!
I’m new using Unity 5.0 and I started c# recently. I’ve created a small script for the enemys in my game who allows them to follow the player. But I want to increase their speed, adding a speed variable. I want to keep quite the same script. Here is the script :

using UnityEngine;
using System.Collections;

public class EnemyIA : MonoBehaviour {

enum dir
{
	LEFT,
	RIGHT
};

GameObject Playerbase;

public int life = 3;

dir direction = dir.LEFT;

// Use this for initialization
void Start () 
{
	Playerbase = GameObject.FindGameObjectWithTag ("Player");
}

// Update is called once per frame
void Update () 
{
	if (Mathf.Abs (transform.position.x - Playerbase.transform.position.x) > 3.5f)
		return;
	dir tmp = direction;
	
	transform.position = Vector3.MoveTowards (transform.position, Playerbase.transform.position, 0.01f);
	if (transform.position.x - Playerbase.transform.position.x > 0)
		direction = dir.LEFT;
	else
		direction = dir.RIGHT;
	
	if (tmp != direction)
	{
		transform.Rotate(0, 180, 0);
	}
}

Sorry for the mistakes I’m french. Thank you.

umm, can’t you do this:

float step = speed * Time.deltaTime;
        transform.position = Vector3.MoveTowards(transform.position, target.position, step);

you can multiply the Time.deltaTiem by the speed variable you want