using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(MyPrefixReplace("12.345.6789", "000"));

        Console.WriteLine(MyPrefixReplace("908.345.6789", "1"));

    }

    static string MyPrefixReplace(string source, string value, char delimiter = '.')

    {

        var parts = source.Split(delimiter);

        parts[0] = value;

        return String.Join(delimiter.ToString(), parts);   

    }

}