1. An apparatus for updating installed software, comprising: (a) a memory that stores different versions of software, including old version software to be updated and intermediate version software that is newer than the old version software; and (b) a processor to execute a process comprising: obtaining version information describing specified new version software; calculating a full update time, a partial update time, and a copy and update time, based on the obtained version information describing the new version software and version information describing the old version software: the full update time being a time required for a full update process that rewrites the old version software entirely with the new version software, the partial update time being a time required for a partial update process that updates the old version software by using a first set of difference files, and the copy and update time being a time required for a copy and update process that first copies the intermediate version software to a storage area accommodating the old version software and then updates the copied intermediate version software to the new version software by using a second set of difference files; selecting a fastest update process from among the full update process, partial update process, and copy and update process by comparing the calculated full update time, partial update time, and copy and update time with each other; and executing the selected fastest update process to update the old version software to the new version software.