This is many times done using the command bpexpdate
# bpexpdate -help
bpexpdate: -m <media id> -d <mm/dd/yyyy HH:MM:SS | 0 | infinity>
[-host <name>] [-force]
[-M <master_server,...,master_server>]
bpexpdate: -deassignempty [-m <media id>] [-host <name>] [-force]
[-M <master_server,...,master_server>]
bpexpdate: -backupid <backup id> -d <mm/dd/yyyy HH:MM:SS | 0 | infinity>
[-client <name>] [-copy <number>] [-force]
[-M <master_server,...,master_server>]
bpexpdate: -recalculate [-backupid <backup id>] [-copy <number>]
[-d <mm/dd/yyyy HH:MM:SS | 0 | infinity>] [-client <name>]
[-policy <name>] [-ret <retention level>] [-sched <type>]
[-M <master_server,...,master_server>]
legal values for sched: 0=full,1=differential incr,2=user,3=arch
4=cumulative incr