Press

(Main Menu) → [Fax/I-Fax Inbox] → [Confidential Fax Inbox].
Select the desired inbox.
Select a file to check → press [Details].
Press

(Main Menu) → [Fax/I-Fax Inbox] → [Confidential Fax Inbox].
Select the desired inbox.
Select the file that you want to delete → press [Delete].
Press [Yes].