You can return a library book at any point before the item's due date by accessing the book's info page and pressing the return button. This will return the loan to the library, and delete the book (and your notes and highlights) form the Bluefire Reader app.
From the library view:
Use the toggle buttons to switch the library into list view
Press the arrow button to the right of the book, this will take you the item information pagePress the return button
From the in-book menu: (while reading,)
Tap the center of the screen once
Press either the Bookmarks or Contents buttonPress the info button in the top center of the screen
Press the return button