upgrade
The method to update the minor engine version. Default value: Immediate. It is valid only when upgrade_db_instance_kernel_version = true. Valid values:
Immediate: The minor engine version is immediately updated.
MaintainTime: The minor engine version is updated during the maintenance window. For more information about how to change the maintenance window, see ModifyDBInstanceMaintainTime.
SpecifyTime: The minor engine version is updated at the point in time you specify.