If it's a booking you've received from a booking portal, like Airbnb, Booking.com, Expedia, etc., you should cancel it from their Extranets/Partner tools. Once you've done so, Smoobu will be notified and the booking will disappear from your Cockpit and the dates will become available on all connected portals.
If it's a direct booking, blocked period or a booking from your website, you should do so from Smoobu.
Option 1) Cockpit page
Go to your Cockpit page (or direct link from here) , scroll down until your Calendar and search for the booking you wish to cancel.
Click on it and it opens the list with the entry Cancel, like this screenshot below:
Option 2) Bookings page
Go to the Bookings list on the left menu (or direct link from here) and look for booking. You might want to use the filters or the search bar.
Once you have found it, click on the Guest's icon or name (in my example, Maria Romana) and Cancel.