I've never done that step myself but it sounds reasonable since you don't want to put the router in an unexpected state while updating the firmware. You can disconnect them forcefully by deleting their IP leases using the router's DHCP settings page and disabling the WiFi network (or change its settings so the computers fail when trying to automatically reconnect).
Secondly, you should always update router firmware using a direct, wired connection between the PC and the router. WiFi connectivity is more prone to errors in packet transfer and a corrupted firmware package will potentially brick the router. Do NOT use a WiFi connection to update the firmware.