Abstract: Operating Systems (OS's) of the same form are stored in first and second memory areas. The executing Operating System in the first memory area is terminated and the Operating System in the second area is initiated. After system files stored in the first area are released from access lock, they are replaced by substitute files provided in advance by using a file replacing function of the second Operating system. When such a file replacement fails for some reason, the original operating system files are immediately restored by using a backup file. By this invention, the updating of files previously forbidden by the Operating System in the first memory area to be accessed as well as the restoration of files at the time of update failure can be achieved by software.