The Network IPS appliance deletes update packages after you install an update. Rolling back an update removes the last intrusion prevention update that is installed on the appliance. When a new update is available, the appliance applies any missing updates, including updates that are not applied to the appliance. If you roll back from the new update, the appliance rolls back to the version that was last applied.
If you install version 1.1 but do not install version 1.2, when you install version 1.3, the appliance adds version 1.2 with version 1.3. If you roll back from version 1.3, the appliance rolls back to version 1.1. It does not roll back to version 1.2 because you did not originally apply that update version.