Go to media and device management, media, volume pools and then none and chose the tape you want to change, right click on it and delete it, grab it out of the library, remove the tape label and put a new one on it, now re-import it into the library as a cleaning tape.
For tapes that have no data on them (all data is expired on the tape) you have to eject them from the library to do this go to media and device management, media, volume pools and then the volume pool you wish to select a tape from and right click on it and then choose change, in the change volume window move it to spare_tapes, now go to the spare_tapes volume pool and eject the choosen tape, once its ejected you can remove it from the library.
Go Gators!
JD