1. An apparatus for updating installed software, comprising: (a) a software storage unit storing different versions of software, including old version software to be updated and intermediate version software that is newer than the old version software; (b) a version information collector that obtains version information describing specified new version software; (c) a processing time calculator that calculates a full update time, a partial update time, and a copy & update time, based on the obtained version information describing the new version software and version information describing the old version software, wherein: the full update time is a time required for a full update process that rewrites the old version software entirely with the new version software, the partial update time is 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 & update time is a time required for a copy & 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; (d) a process selector that selects a fastest update process from among the full update process, partial update process, and copy & update process by comparing the calculated full update time, partial update time, and copy & update time with each other; and (e) an updating unit that executes the selected fastest update process to update the old version software to the new version software.