char message[] = "0A0100004006C03F";
uint64_t messageValue;

void setup() {
  Serial.begin(115200);
  messageValue = strtoull(message, nullptr, 16 ); // https://cplusplus.com/reference/cstdlib/strtoull/

  byte *ptr = (byte *) &messageValue;

  // print what we got in memory, reverse order because of little endian representation 
  for (int i = (sizeof messageValue) - 1 ; i >= 0; --i) {
    Serial.print("0x");
    if (ptr[i] < 0x10) Serial.write('0');
    Serial.println(ptr[i], HEX);
  }
}

void loop() {}