If your changer has its own SCSI ID

If your changer has its own SCSI ID, separate from the tape drive, the Device Management window looks similar to this:

Select the changer by clicking on it.
To perform any device management functions with your changer, just click anywhere on the box next to the SCSI ID of the changer.
If you click any other box, such as the ones showing the standard tape drives, you will see the standard device management options.