In the host panel you can choose the listing for which you want to change the cancellation conditions. Click on the pencil-icon (on the top right) to have access to edit details. In the edit mode there is a tab with Booking Settings. Here you can specify the maximum days a guest can cancel the booking prior to the start date. When you have adjusted this to the desired number of days, you can save the edit and it will automatically become active for your listing.