var myString = string.Format("Business Invitation, start time, M Problem, 518-06-xxx, 9999 999 0783, {0}, xxx ??", yourMeetingId);

var startingIndex = myString.IndexOf("0783, ");
var endingIndex = myString.IndexOf(",", startingIndex + 6);
var pattern = myString.Substring(startingIndex + 6, endingIndex - (startingIndex + 6));
myString = myString.Replace(pattern, yourMeetingId);

string pattern = @"d{3},";
    Regex regex = new Regex(pattern);

    var inputStr = "518-06-xxx, 9999 999 0783";
    var replace = "..."
    var outputStr = regex.Replace(inputStr, replace);

string temp = "Business Invitation, start time, M Problem, 518-06-xxx, 9999 999 0783, Meeting ID, xxx ??";
        string newID = "1234";
        string[] firstSplits = Regex.Split(temp, "0783,");
        string[] secondSplits = Regex.Split(firstSplits[1], ",");
        secondSplits[0] = newID;
        string @join = string.Join(",", secondSplits);
        firstSplits[1] = @join;

        string newString = string.Join("0783,", firstSplits);