>I have 20 old cartridges and all are having data.
>Now can i delete or erase the data from all these tapes
>and reuse it again for new backups.

You didn't specify what those tapes are...whether foreign or
in a stgpool.  If in a stgpool, they will be implicitly reused
when their contents expire or are moved off to another volume.
If they are foreign tapes, outside the library, then you should
do a Label Libvolume to introduce them to your scratch pool.

    Richard Sims,  http://people.bu.edu/rbs