Mid$(sOutput, 1) = EncodeQuantum(b)

 Sub Main()
        Dim input As String, startIndex As Integer

        input = "Some test string"
        startIndex = 5
        Mid$(input, startIndex + 1, 13) = "new string123"

        Console.WriteLine(input)
        Console.ReadKey()

        'result:
        'Some new string1

    End Sub

static void Main(string[] args)
        {
            string input = "Some test string";
            int startIndex = 5;
            string replStr = input.Substring(startIndex);
            input = input.Replace(input.Substring(startIndex), "new string123".Substring(0, replStr.Length));

            Console.WriteLine(input);
            Console.ReadKey();

            //result:
            //Some new string1
        }

Dim sOutput, replString

sOutput = "123456789"
replString = "Repl"

Mid$(sOutput, 7, 4) = replString

static void Main(string[] args)
        {
            Console.WriteLine(Mid("123456789", "Repl", 7, 4));
            Console.ReadKey();
        }

public static string Mid(string inpStr, string replStr, int startIndex, int longIndex)
        {
            if (longIndex > 0)
            {
                --startIndex; //для уравнения стартовых индексов VB & C#
                int inpStrLen = inpStr.Length;
                int replStrLen = replStr.Length;
                if (0 <= startIndex && startIndex <= inpStrLen)
                {
                    int len = inpStrLen - startIndex;
                    string str1 = String.Empty;
                    string str2 = String.Empty;

                    if (len > replStrLen)
                    {
                        str1 = inpStr.Substring(startIndex, longIndex > replStrLen ? replStrLen : longIndex);
                        str2 = replStr.Substring(0, longIndex > replStrLen ? replStrLen : longIndex);
                    }
                    else if (len < replStrLen)
                    {
                        str1 = inpStr.Substring(startIndex, inpStrLen - startIndex > 0 ? len  : longIndex);
                        str2 = replStr.Substring(0, inpStrLen - startIndex > 0 ? len : longIndex);
                    }
                    else
                    {
                        str1 = inpStr.Substring(startIndex, len);
                        str2 = replStr.Substring(0, len);
                    }
                    return str1.Equals(str2) ? inpStr : inpStr.Replace(str1, str2);
                }
                else return String.Empty;
            }
            else return inpStr;
        }