There doesn't seem to be any mention in the Nexentastor-API documentation of how to go about upgrading the appliance itself. There's mention of a:
(int32) upgrade_check (int32 invalidate) Check for appliance software upgrades
And the possibility of activating an upgrade checkpoint:
System Upgrade, Rollback and Checkpoint Management
But is there a method to initiate the download of the update itself?
Unfortunately NMS does not have any methods to do upgrade.