PlatformScript.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlatformScript : MonoBehaviour {

    public GameObject arrow;

    public bool isHorizontal = true;
    public float speed = 2f;

    public float min;
    public float max;

    bool directionPlus = true;

    void Start() {
        SetDirection(true);
    }

    void SetDirection(bool dir) {
        directionPlus = dir;
        float angle = isHorizontal ? (dir ? 0 : 180) : (dir ? 90 : 270);
        arrow.transform.eulerAngles = new Vector3(arrow.transform.eulerAngles.x, arrow.transform.eulerAngles.y, angle);
    }

    void FixedUpdate() {
        if (isHorizontal) {
            if (transform.position.x > max) {
                SetDirection(false);
            } else if (transform.position.x < min) {
                SetDirection(true);
            }
        } else {
            if (transform.position.y > max) {
                SetDirection(false);
            } else if (transform.position.y < min) {
                SetDirection(true);
            }
        }

        float delta = speed * (directionPlus ? 1 : -1) * Time.deltaTime;

        if (isHorizontal) {
            transform.position = new Vector3(transform.position.x + delta, transform.position.y, transform.position.z);
        } else {
            transform.position = new Vector3(transform.position.x, transform.position.y + delta, transform.position.z);
        }
    }
}