public float m_TotalTime = 5f;

    private float m_changeSpeed = 0;
    private Image m_bgImg;

    private void Start()
    {
        m_bgImg = GetComponent<Image>();
        m_changeSpeed = 1 / m_TotalTime;
        StartCoroutine(ShowCountingDown());
    }

    float timer = 0f;
    IEnumerator ShowCountingDown()
    {
        while (m_bgImg.fillAmount > 0)
        {
            m_bgImg.fillAmount -= m_changeSpeed * Time.deltaTime;
            yield return new WaitForSeconds(Time.deltaTime);
        }
        StopCoroutine(ShowCountingDown());
    }