Here is a simple solution to your question. I took each character, checked against its hexadecimal character and returned as a combination.

char CardNumber[8] = "B763AB23";
byte CardNumberByte[4];

void setup() {
  Serial.begin(9600);
}

void loop() {
  for(char i = 0; i < 4; i++)
{
  byte extract;
  char a = CardNumber[2*i];
  char b = CardNumber[2*i + 1];
  extract = convertCharToHex(a)<<4 | convertCharToHex(b);
  CardNumberByte[i] = extract;
  Serial.println(CardNumberByte[i], HEX);
}
  
  while(1);
}



char convertCharToHex(char ch)
{
  char returnType;
  switch(ch)
  {
    case '0':
    returnType = 0;
    break;
    case  '1' :
    returnType = 1;
    break;
    case  '2':
    returnType = 2;
    break;
    case  '3':
    returnType = 3;
    break;
    case  '4' :
    returnType = 4;
    break;
    case  '5':
    returnType = 5;
    break;
    case  '6':
    returnType = 6;
    break;
    case  '7':
    returnType = 7;
    break;
    case  '8':
    returnType = 8;
    break;
    case  '9':
    returnType = 9;
    break;
    case  'A':
    returnType = 10;
    break;
    case  'B':
    returnType = 11;
    break;
    case  'C':
    returnType = 12;
    break;
    case  'D':
    returnType = 13;
    break;
    case  'E':
    returnType = 14;
    break;
    case  'F' :
    returnType = 15;
    break;
    default:
    returnType = 0;
    break;
  }
  return returnType;
}

Here is a simple solution to your question. I took each character, checked against its hexadecimal character and returned as a combination.

char CardNumber[] = "B763AB23";
byte CardNumberByte[4];

void setup() {
  Serial.begin(9600);
}

void loop() {
  for(char i = 0; i < 4; i++)
{
  byte extract;
  char a = CardNumber[2*i];
  char b = CardNumber[2*i + 1];
  extract = convertCharToHex(a)<<4 | convertCharToHex(b);
  CardNumberByte[i] = extract;
  Serial.println(CardNumberByte[i], HEX);
}
  
  while(1);
}



char convertCharToHex(char ch)
{
  char returnType;
  switch(ch)
  {
    case '0':
    returnType = 0;
    break;
    case  '1' :
    returnType = 1;
    break;
    case  '2':
    returnType = 2;
    break;
    case  '3':
    returnType = 3;
    break;
    case  '4' :
    returnType = 4;
    break;
    case  '5':
    returnType = 5;
    break;
    case  '6':
    returnType = 6;
    break;
    case  '7':
    returnType = 7;
    break;
    case  '8':
    returnType = 8;
    break;
    case  '9':
    returnType = 9;
    break;
    case  'A':
    returnType = 10;
    break;
    case  'B':
    returnType = 11;
    break;
    case  'C':
    returnType = 12;
    break;
    case  'D':
    returnType = 13;
    break;
    case  'E':
    returnType = 14;
    break;
    case  'F' :
    returnType = 15;
    break;
    default:
    returnType = 0;
    break;
  }
  return returnType;
}