1. An update apparatus used in a system that provides service using software and performing update of the software, the update apparatus comprising: a processing apparatus configured to calculate a difference between a possible down time of the service according to a service level and a down time of the service indicated by operation performance stored in storage unit, and when there are a plurality of pieces of software as update targets, to calculate an update order of the plurality of pieces of update target software according to a degree of influence to the difference by service down when update of software fails, and to perform update of the plurality of pieces of update target software according to the update order.