string a = "12-34-56";
string b = a.Remove(a.LastIndexOf('-'),1);
Console.WriteLine(b.Replace('-','.'));

string myString = "2-34-76";
        Console.WriteLine(myString);

        myString = myString.Replace("-", string.Empty);
        Console.WriteLine(myString);

        myString = myString.Insert(myString.Length - 4, ".");
        Console.WriteLine(myString);

        Console.Read();
    }

var value = "2-34-76";

var firstInstance = value.FirstIndexOf("-");

var newValue = value.Substring(0, firstInstance+1) + "." + value.SubString(firstInstance+1);

string text = "5-87-34";
string formatted = text.Insert(text.IndexOf('-'), ".").Replace("-",string.Empty);