Files can be deleted from your account online. To do this hover the mouse over the file you wish to delete and click delete on the right side of the highlight.
Deleting folders containing no other folders will update your space available and therefore free up backup space, deleting folders that do contain other folders will not update your space available.
What we advise doing is ensuring in your backup selection you only include files and folders you are sure you want backed up. If you need to delete a folder that contains other folders, and want it to reflect in your space available, you can still do so - you will need to delete from the bottom level folder upwards.