using System;

using System.Linq;

​

// replace pin with your pin

string input = "00:1E:35:3B:7E:6D";

​

​

var chars = input

    .Split(':')

    .Select(x => (char)Convert.ToInt32(x, 16))

    .Reverse()

    .ToArray();

​

var result = new string(chars);

Console.WriteLine(result);