To modify the retension of either backup or archive

Archive retension:

update copygroup domain_name policy_set_name class_name type=Archive destination=pool_name RETVer=days

(Note: if you don't need to modify destination then skip the destination=pool_name)

Backup retension:

update copygroup domain_name policy_set_name class_name type=Backup VERExists=Number VERDeleted=number RETExtra=days RETOnly=days