No cancel button. Read the online documentation. To cancel you have to delete all your computers from your profile which deletes the underlying backups.
The yearly charge will be authorized in the days before your subscription expires “so there’s no interruption in service.” But you can’t tell them you want to cancel without deleting all your backups and you don’t know when the charge is going to renew.
So in order to use you’re full subscription you’ll need to set an alarm to cancel on that day, and then go ask for a refund because they’ve already charged you for the next year.
It’s an intentional dark pattern to make cancelling difficult.