1. An update apparatus used in a system that provides service using software and performing update of the software, the update apparatus comprising: a processor configured to calculate a first 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 a storage unit, and when there are a plurality of pieces of software as update targets, to calculate a plurality of second differences, each of the plurality of second differences comprising a difference between the first difference and a down time of the service when an update of one of the plurality of pieces of software fails, to set an update order of the plurality of pieces of update target software to be a descending order of the second differences, and to perform an update of the plurality of pieces of update target software according to the update order.