If the booking is Non-Refundable, it is not possible to change the room type booked.
If the booking is Refundable, you can cancel it free of charge and made a new booking for the room you wish.
Please note it is not possible to keep the original rate and pay only for the difference between the rooms.
However, you can try to contact the hotel directly and check for a room upgrade.
You can contact the hotel from the booking confirmation sent to you - at the top of the booking confirmation choose 'Contact The Property'.