Click "My Files" in your file manager which will display all the files and folders in your account.
Navigate to the folder you want to delete and select it by clicking the checkbox next to it. You can also click anywhere else on the row to select it.
Then RIGHT CLICK to bring up the list of options and then click "Delete".
Note: Deleting a folder will permanently delete ALL files and subfolders within that folder!
Tip: You can select many different folders at once and delete them all at the same time.