That's not an encode or decode: all you are doing is changing the number base.
This has nothing to do with big-endian or little-endian: all you are doing is selecting to output a number in base 10 or 16.
Provided your decimal number does not exceed Int64.MaxInt, all you need to do is
Int64 i = 0;
string dec = "3569869142";
string hex = "Input number conversion failed";
if (Int64.TryParse(dec, out i))
{
hex = i.ToString("X");
}
Console.WriteLine(hex);