Please note that firmware upgrade using Web GUI is only possible within the same firmware series, e.g. from 1.6.x to 1.6.z.
To upgrade from version 1.6.x to 1.7.z please use the NVR interface. 
Alternatively you can copy the firmware file onto a USB flash drive, plug it into the NVR and reboot the NVR. The NVR will automatically upgrade during bootup. 