If you'd like to change or cancel your flight, contact the airline 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 airlines are currently offering fare flexibility.
The confirmation email(s) that you received after booking your flight typically contain the confirmation number and the airline or travel agency's contact information. You may also find more information in the details of the corresponding trip on google.com/travel.