Start the NetBackup Administration Console.  (see relevant doc)

From the window on the far left, in the Media and Device Mangagement
subheading, click on Media.

Click on the tapes you wish to remove.  Hold down the Shift key while
clicking to highlight multiple tapes in a row, or hold down the Control
key to highlight multiple, non-sequential tapes.

Right-click on the highlighted tapes and select Eject Volume(s) From
Robot from the pop-up menu that appears.  A dialog window will open to 
confirm that you wish to eject those tapes.  Click on the Eject button
in the dialog window.

The dialog window will indicate that the tape eject is occurring.  If
you are ejecting more than 2 tapes, then you will need to remove the tapes 
as they arrive in the Media Access Port of the robotic tape library.  The
Media Access Port has only two slots for tapes, so you need to remove the
tapes from the Media Access Port in order to continue ejecting more tapes.
A dialog window will appear, indicating that you need to remove tapes from
the Media Access Port.  When yo have done so, click Yes to continue 
ejecting tapes.

To physically remove tapes from the library:

NOTE: The Media Access Port is called the CAP (Cartridge Access Port) on
the library display.

The library display should read "CAP: SELECT to open"

Press the SELECT button.  The CAP (aka Media Access Port) will pop
open.  Pull it open all the way and remove the tape(s).  Then push
the CAP closed.