If you are running FTD image, a Firepower 2100 series runs exclusively in Appliance mode. In that mode, there is no FCM GUI. The hardware management interface is used by the FTD image as the logical device management interface (unlike the 4100 and 9300 series which uses separate physical interfaces).
You copy/install new FTD software using the management GUI - either Firepower Device Manager (local management) or FMC (remote management).