@Vicky68
Vicky - the steps are as follows - you guest needs to cancel or enter the new departure date on the platform. It will then be notified to you as a cancellation and/or change to dates. For a change to dates, you would need to accept the change for it to be valid.
AirBnB will then apply the long term canceallation policy. You don't need to do anything. Once you have been paid you can decide if you wish to return some or all of the money - or you could tell the guest that you are happy to refund the nights you manage to sell following their departure.