A technique that guarantees update of a program of a finisher unit while preventing a malfunction thereof from being caused by erroneous attachment thereof to a printing apparatus. Hardware identification information is acquired from the finisher unit. When it is determined based on the acquired hardware identification information that the finisher unit is compatible with a program of the apparatus, program identification information is acquired from the optional unit, for determining based thereon whether a program of the finisher unit program is associated with the program of the printing apparatus. If the programs are associated with each other, the versions there are compared. If the versions are not compatible, the program of the finisher unit is updated to a version compatible with the version of the program of the printing apparatus.