Convert string to hex number

Hexadecimal, octal, decimal, binary: they are only representations of the same number. There is no hex number 4E, there is only the value 78. You can parse the hexadecimal representation to get this value, and thank back again to your desired format - but do you really need this?
Check following code lines one by one:

var input = "4E";
var x = Convert.ToInt32(input, 16);
Console.WriteLine(String.Format("&H{0:X}",x));
Console.WriteLine(String.Format("&H{0}",input));