To convert Decimal or Integer value to Hexadecimal you can go this way..

byteRead = Serial.read();  //Read your value
myValue =  String(byteRead, HEX); //Covert it into Hexdecimal   
Serial.println(myValue); //Print in serial Monitor 

To convert Decimal or Integer value to Hexadecimal you can go this way..

byteRead = Serial.read();  //Read your value
myValue =  String(byteRead, HEX); //Convert it into Hexadecimal   
Serial.println(myValue); //Print in serial Monitor 
myValue1 = String(byteRead,DEC); //Convert it ino Decimal
Serial.println(myValue1); //Print in serial Monitor