i took symantec support for changing my Exchange GRT retention time, as we have accidently given 1 week retention time to our weekly full GRT Exchange backup on disk Then we Symantec engineer change this to one month .
My environment was
Netbackup Master server 126.96.36.199
Netbackup Client (Exchange 2010) version 188.8.131.52
they use the command bpexpdate
i took screen shot also
check this C:\ProgramFile\Vertitas\Netbackup\bin\admincmd\bpexpdate -recalculate -backupid <grtclient_1234567890> -ret 3