Making an object move back and forth in a 2D platformer.

I can't figure out how to make an enemy move back and forth on the x-axis.

This is what I am attempting to do in the Update() method of the EnemyStats script:

 patrolTimer = Time.deltaTime;
 
         if (patrolTimer >= 0) 
         {
             GetComponent<Rigidbody2D> ().velocity = new Vector2 (speed, GetComponent<Rigidbody2D> ().velocity.x);
             patrolTimer = 0;
         }
         */
 
         /*
         if (patrolTimer >= 4)
         {
             GetComponent<Rigidbody2D> ().velocity = new Vector2 ((-1 * speed), GetComponent<Rigidbody2D> ().velocity.x);
             patrolTimer = 0;
         }
         */