1. Shanmuga gives good advice for considering creating a duplicate copy of the image. NetBackup, by default does not allow mixed retention levels on a single tape. If you have a tape that contains mostly 1 month retention and you increase retention to 1 year on one of the images then the earliest the tape containing mostly 1 week retentions will expire will now be 1 year. Do you see how this could possibly cause you to use more tapes if you practice extending retentions regularly?
2. Shanmuga brings up good point that you may want to consider the retentions before doing the backups.
3. Karthikeyan provides the syntax and example of how to change the retention from the command line. There is an online man page available on the bpexpdate command and it is also described in the "Commands for UNIX" or "Commands for Windows" manuals. I do not know if you can access the online man pages so you may want to download the pdf file to your desktop.