private int[] RepalceArray(int[] values, int pos, int oldvalue, int newvalue)
            {
                values[values.Select((a, i) => new { i, a }).Where(a => a.a.Equals(oldvalue)).Take(pos).Reverse().FirstOrDefault().i] = newvalue;
                return values;
            }

------解决方案--------------------
C# code
        string oldarr = "1,2,3,4,5,5,6,7,4,3";
        int arrnum = 3;
        string newstr = "5";
        string[] parts = oldarr.Split(',');
        parts[arrnum] = newstr;
        string r = string.Join(",", parts);
        Response.Write(r);