When Backup Exec runs out of usable tapes, it will ask you to Insert Media in the Library Portal.
In devices, under robotic libraries right-click on a slot with a full tape and choose Export. When you get the alert to remove media, just remove it from the mailslot. Insert the new overwritable or empty tape
Then right-click on the same slot in the GUI and choose import.
There is a chance that The Export and Import jobs won't work, because they will be queued behind the running job that is requesting media.
In that case you use the Autoloader front panel or web interface to unload a tape and load another tape. Then in Backup Exec you right-click your robot in the Devices tab and choose "Scan". If you have barcodes it should pick up the new tape. Then you can acknowledge the alert to "Insert new media" and the job should continue.
There is a slight chance this won't work either. But come back to us if this second option doesn't work.