@Sarah3660 , are you working from a laptop? Go to your Airbnb inbox and find the conversation with the host. On the right side of the screen you should be able to scroll down until you see "Change or cancel reservation". Click that and follow the prompts.
Or you can do it though Trips. Look in "upcoming" for the reservation. Click on the photo, then click it one more time and you will see the details and "change or cancel" will be on one of the lines.