@Gloria34 go to Manage Listing and click on the menu on the upper left of the page. Select Check - In and you will see a place to add / change your cancellation policy.
I'm accessing Airbnb through the desktop site, so it could be different if you're on the app. You should be able to edit the policy even if you haven't listed it yet.
Go to Manage Listings > Listing Details > scroll down to the Cancellation Policy area (after About this space, description, amenities, house rules, etc).