string str = "ABC00001";
            str = str.Substring(0, str.Length - 5) + "FFFFF";

------解决方案--------------------
C# code

/// <summary>
        /// 测试
        /// </summary>
        /// <param name="value"></param>
        /// <returns></returns>
        string Demo(string value)
        {
            if (value.Length < 5)
            {
                return;
            }
            return value.Substring(value.Length - 5) + "FFFFF";
        }