You don't, Backup Exec does it automatically. Basically you set the job to point to a certain media set, and Backup Exec will then select a tape to be used to perform the backup (assuming a tape library, otherwise it'll use the currently inserted tape), assuming there is a tape which is available to be written to. When it does the backup, it will then automatically move the tape into that media set, which then determines the overwrite and append protection period applied to that tape.
So basically at a later date if you had the same tape available for writing to it and ran another backup job which pointed to a different media set, the tape would be moved to that new media set, rather than staying in the first one.
