Source: http://www.adsm.org/lists/html/Veritas-bu/2001-10/msg00155.html
To change the off-site tapes expiration date from 3 months to 1 year from
the original backup date you can use one of 2 methods. The first is the
easiest and works for our setup because we want all images on a media to be
extended. If you want to do just one image on a media then you can use
second method.
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Method 1) – All Images on a Media
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bpexpdate -ev -d
ie.
# bpexpdate -ev 003363 -d 03/25/2002
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Method 2) – A Single Image on a Media
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The following three steps describe changing the retention level or
expiration date on duplicate images.
1) Make the duplicate image the primary copy to work with:
/usr/openv/netbackup/bin/admincmd/bpduplicate -npc 2 -backupid
2) Set a new expiration date for the duplicate image:
/usr/openv/netbackup/bin/admincmd/bpexpdate -recalculate -d -backupid
-- Thiers Family -- 
Thank you. it worked like a charm