You can make the drive letter anything you want it to be, between A-Z as long as there is not a drive with the same letter already existing. Obviously you don't want to use A, B or C as those are normally the floppy drives and primary hard drive.
You don't mention your OS so I can't tell you how to reassign it without that bit of information.
BTW, when I setup a computer I will often make the first CD drive E or higher. That way if I decide to install another hard drive later then my CD will not be reassigned and my programs that look for it at a drive letter will work without any reconfiguration.