If you'd like to change or cancel your reservation, contact the hotel or online travel agency directly for help. Google is unable to directly assist with changes and cancellations.
Make sure that you check the cancellation policy so that you're aware of any cancellation-related deadlines or penalties, and whether you'll receive a refund. Some hotels are offering flexible policies for hotel reservations.
The confirmation email(s) you received after booking a hotel typically contain the confirmation number and the hotel or travel agency's contact information. You may also find more information in the details of the corresponding trip on google.com/travel.